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Lane

Lane's: jtallen83 inspired 21.750" 300 AAC Blackouts

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I don’t have anyway to check the accuracy myself. However, The Firearm Blog had an article that compared it to an expensive commercial unit. It averaged about +- 2fps of the big boy. 

https://www.thefirearmblog.com/blog/2017/06/30/labradar-vs-professional-weibel-doppler-radar-benchmark-test/amp/

I think that’s pretty darn good for something that the average Joe can afford. At 2000 FPS, that’s what...0.1% error. 

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39 minutes ago, SimonSays said:

It averaged about +- 2fps of the big boy. 

That's fine for this purpose of course. It was the stated "better than 0.1% error" that I saw in the specs, and wondered just how much better it really was.

I spent a bit of time looking over discussions about QuickLoad. It appears the powder I have on hand (CFE BLK) isn't even in their latest database update. That's actually not the worst thing though; it means I will have to do my initial load testing by hand, and then try to hit OBT numbers from empirical data. The charge weights in most 300 Blackout loads are near 1.0 grain between start and max loads. Ladder loading shouldn't take very long... In a technical sense, it seems more appropriate to gather data first; and then see if the calculations match up. This is a good case for a shoot first and ask questions later approach. Of course I will still use QuickLoad later on to work up loads with other powders when I get to that point.

Did some more reading about the failures of Long's theory; and there is some dissent in regards to totality and accuracy. Some people didn't get matching data with an explicit barrel length, and others found correlation; but also intermediate nodes that the theory doesn't address. Someone explicitly mentioned having to add exactly 1.5" to the barrel length to make it return the correct values. Seems like a great theory with plenty of room to grow.

This thing showed up in the mail today; doesn't look like much, but it's a 10 GHz doppler radar module. I don't think it will help me inspect bullet speeds, but it's worth looking at in my spare time. Initial investigation indicated it could probably be improved dramatically with a horn antenna; but those are well over $100. I noticed one reviewer of the labradar indicated it didn't work at all where his targets were near trees. I wonder if I would be in that same position here; I shoot off into the edge of a wooded area myself.

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Kolbe and Vaughn showed up in some of these discussions about vibrations and OBT theory. This is a similar physical tuning based on the same basic principals (trying to time the bullet exit to barrel flop). In this case a weight was attached to the end of the barrel to change it's resonant frequency; slowing wave propagation slightly to achieve an idea exit. Vaughn used an accelerometer to look at the same type of movements at the end of the barrel.

http://www.geoffrey-kolbe.com/articles/rimfire_accuracy/tuning_a_barrel.htm

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That X Band radar is pretty darn cool. What would your pulse width and pulse repitition rate be? Any idea what your beam width or receiver gain needs to be to detect a bullet? >$100 for a horn antenna is nuts. They’re not that hard to make. 

Are you an EE or just really handy with hardware and software?

Sorry for all the questions. Let’s make it easy. Just send in an application, resume and $50 processing fee to me. 😉

I haven’t read your links yet but I will ASAP. 

My shooting range has a “short” lane for up to 300 yards, 600 yard mid and 1k long. I’ve had no problems from the Labradar detecting my rounds because of scatter. However, I can see it having trouble getting a reading if your target was stapled to a heavy tree line at 50 or 60 yards. That’s well within it detection range and I could see the heavy reflection from the trees causing the receiver to loose track. Just a guess. 

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Another thought. If that radar can detect a bullet but not it’s speed (even though it is a Doppler) you could use 2 to replace the optical sensors in a home made chrono. 

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https://royalsocietypublishing.org/doi/pdf/10.1098/rspl.1901.0052

This is ancient history; published in 1901 discussing the same basic principles of vibration in a gun. Notice how the vibration is carried across the entire rifle. This quote from the beginning of the paper; emphasis added by me in terms of the addition of the stock to the resonance measurement.

Mallock Quote: "The problem of “ jump ” may be stated mathematically thus an elastic tube, to which a mass is unsymmetrically attached, is subjected for a given time to a couple of arbitrary magnitude. Determine the subsequent motion.” To solve this problem we must consider the tube and its attached mass as forming a single system, and examine w at are the natural modes of vibration of this system, and what their natural periods."

Interesting to note that jump is always used in quotes to designate ambiguity.  Later described in this paper is the issue that "jump" turns out to mean more than one thing depending on the context. I've certainly misinterpreted it in some other reading once or twice; quoted below it states that "jump" can also refer to the more common definition of bullet seating in the rifling of the barrel.

Mallock Quote: "There is another form of “ jump,” however, in the Lee-Enfield rifle, whose absence is most desirable, as it introduces horizontal movements of the barrel; depends, not on the acceleration of the shot, but on the statical pressure of the powder gas acting on an asymmetrical breech-closing action, and the remedy as well as the disadvantages are so clear in this case as not to call for further remark."

The attached pressure trace images shows the difference between seating in the lands, and 30 thousandths of an inch back. I'm going to go out on a limb and suppose that the 21.750" might still might hold weight given the depth of prior research before the Houston Warehouse experiments went down. Unfortunately it would be difficult to approximate the resonance of those actual guns vs. an AR build. The idea of adding weight to the end of the barrel for tuning might be extremely useful in terms of playing those numbers in a non-destructive way. 

This is another reason I was scared of adding gas blocks to these builds; it would adjust resonance in an unpredictable way (the torque on screws would adjust the coupling among other variations). 

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Just now, SimonSays said:

Another thought. If that radar can detect a bullet but not it’s speed (even though it is a Doppler) you could use 2 to replace the optical sensors in a home made chrono. 

Good call, and I've got two of another style doppler module that are just that; designed for detecting an object in close proximity. Would have huge advantages over even the best optical setups. This actually makes sense in the short term to get fast cheap results. 

I was always planning to stretch out my optical chrono on a 10' 2x4 (while 20' would be better; it seems unwieldily). This would certainly be a better option for that kind of build.

Thanks! :thumbup:

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14 minutes ago, SimonSays said:

Sorry for all the questions. Let’s make it easy. Just send in an application, resume and $50 processing fee to me.

I don't even have a current resume; this is the kind of thing I do for fun mainly. I used to spend a lot of time at the local makerspace volunteering; but I live much further away now. I've ended up buying my own tools and equipment to fill those gaps as a result.

Pulse repetition is specified as 2 kHz. I didn't open the can on the other side myself, but someone else posted pictures online (linked below). It's pretty much a passive device; it can be used to detect speeds of course.

Beam width is 72 degrees (average I think); datasheet says x of 40, and y of 80 degrees I think. Someone mentioned getting something like 80 meter distance with a horn antenna. Couldn't the beam width be adjust with a parabolic antenna (small satellite dishes work fine for 2.4 GHz wireless at least)?

There is a 24 GHz module available (CDM324) with an amplifier built in, and quite a few amplifier designs for this module as well. 

https://www.allaboutcircuits.com/news/teardown-tuesday-hb100-doppler-radar-module/

https://www.limpkin.fr/public/HB100/HB100_Microwave_Sensor_Module_Datasheet.pdf

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5 minutes ago, Lane said:

Pulse repetition is specified as 2 kHz

Can't a few of these modules also be configured in an array; and polled round robin to hit some serious resolution in terms of grabbing data as the bullet leaves the barrel? It would look like a space weapon with five mounted around the end of the barrel; not to mention wiring...

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I'm kind of disappointed in this QuickLOAD issue. That is one of the many complaints about this software I've seen; it doesn't have data for newish powders. Turns out there may be a reason for it; but I can't get data for my 7.62x39 or 300 AAC Blackout right now as a result. 

This was posted in 2015... and talks about the issue more explicitly...

"What Hartmut does in his lab is take a sample of a new powder and measured solid and bulk densities and then burn a fixed weight of the sample in a closed vivacity bomb to get a pressure vs. time curve. Dynamic vivacity is then the rate of increase (the slope of the curve) divided by the peak pressure at the end of the burn. He uses the resulting vivacity at a number of points on the curve and the final pressure reached to get the stored energy and progressivity information about the powder. Where the corners of the curve are and their slopes and total elapsed time let him deduce the other characteristics. I think deducing so many characteristics from one curve is the main innovation Herr Broemel program contributes to powder technology. Hodgon could do that if they wanted to, but they don't have a reason to spend the money, as the information would only be of interest to QuickLOAD users, AFAICT, and why should Hodgdon undertake to support the program for no return on the investment? "

This information is the only way to get the data needed for simulation. Looks like Lil Gun is probably going to be an Ok powder for calculations. But somebody has to be able to measure CFE BLK for less than 50 grand; right? Who wants to build a closed vivacity bomb in their garage? That would look something like this:

http://www.ozm.cz/en/burning-rate-tests/closed-vessel-rb-series/

I can easily work OBT calculations on .308 Winchester, and .223 Remington... But what's the point when those guns of mine are generic builds? ... Yes; we could talk about this OBT theory, and play around with those loads. But my target builds are not in that realm.

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I almost bought powder while I was out today; but have decided to hold off. It would be best to start that search with the list of powders available in QuickLOAD; and the cross check everything between 7.62x39 and 300 AAC Blackout all over again. I want at least two different options that can be used in simulation, at different ends of the burn rate spectrum. This would allow me to at least briefly look at Long's theory on critical point of resonance during powder burn. He also recommends filling as much case volume with powder as possible to avoid problems there; so that's another consideration in powder choice.

Been thinking about Vaughn's use of an accelerometer to measure flap in the barrel as the bullet travels down the tube. While it would have been humorous to strap an iPhone or Wiimote to the end of the barrel; neither device has a high speed accelerometer.  In the high speed department Analog Devices sells a three axis chip that can be polled at 50kHz or better. The datasheet was a bit vague; but it can provide samples at least every 20 microseconds; perhaps as good as every 4.5 microseconds. If that 4.5 µs sample rate can be achieved that's close enough when looking at a bullet traveling 1 millisecond or more down the barrel. That would provide more than 222 samples during the bullet's journey. At $60 for a bare chip I'm not quite ready to order; but it's a contender. I still have to look around to see what higher speed options might be available; and what they would cost.

Another polymer80 lower showed up for this project (I swear; it will be black on the outside when it's done). It was purchased as a blemished unit; and they claimed the blemish wouldn't impact performance. I beg to differ; there was a hole in the casting surrounding where the trigger should be protruding. It would have been visible in final form; and that area is quite thin to begin with on a polymer80. Got some JB Weld Plastic which was roughly what was recommended for this. I plan to mill off the lump before starting the FCG, and go slowly with the smallest bit I have when I punch the trigger hole... I suspect that epoxy will be more brittle than the polymer.

I chose polymer for this project because it will be a good reference in terms of damping the system (and the takedown pins are likely to be VERY tight). I also have an aluminum lower waiting for the same reason; it will damp the vibrations a bit less (and that one is already black).

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1 hour ago, Lane said:

two different options

Apparently I only have two options (so far); but all of these powders have QuickLOAD data of course. Does anyone know if the IMR and H 4198 are technically different? I see different grain weights for them; should I use both and call that three? I probably need to get a few off the partial data list as well in terms of burn rate comparison.

Crosses between 7.62x39/300 Blackout well: Accurate 1680, Both 4198s?

Partial data across one or both cartridgesBL-C(2), H110, Win 296, Benchmark, IMR 4227.

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Now milled, and drilled. The plastic epoxy held up very well; though I still tool my time with a smaller bit in that area.

I had the same problems drilling the selector holes; once the drill bit penetrates it gets stuck in a bow tie shaped hole. I even tried running pilot bits and had the same problems; especially on the side with the detent hole. I simply used a file for cleanup this time; until I could drill holes clean. 

No rush on the final assembly here; but it sure is satisfying to knock one of these out in about an hour. 

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I meant to offer this data the last time I did a polymer lower; but never pulled the pin back out. This is a modified front takedown pin which is longer than a standard rear takedown pin. Because of the casting shape on the receiver, a standard rear pin wouldn't protrude at all. That makes it nearly impossible to actuate without tools.

I start off milling a channel with a tiny bit; then remove it from the vise and do the final finishing by hand. I never feel like I can see well enough to hit the target with all the shavings. That's only 0.035" between the end of the pin and the channel cut. I'm going to fill that old detent with epoxy this time around so it doesn't catch in the wrong place. 

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Read about half of Harold Vaughn's book "Rifle Accuracy Facts". Turns out he used a one dimensional piezo accelerometer, which would be reasonable to acquire. He also took horizontal measurements and found that movement to be about 1/3rd of vertical barrel movement. The same 3rd order harmonics were dominant; something Mallock talked about back in 1901. Vaughn actually advocates damping at the fore end of the barrel to tune out third order vertical vibrations.

I have a whole list of comparisons or interesting facts from what I did read. A lot of good data regarding chamber clearances; and that he observed almost a half inch of throat erosion after a few thousand rounds. He also agrees with Long, that a full case of powder is necessary for accuracy. Some things I skipped, because they didn't apply to me (can't use a muzzle device, scopes can't rattle, etc.). Other things won't matter in these specific builds for other reasons; I'll circle back on those later.

It seems like barrel bedding and nut torque are going to be serious concerns. His experience was that even permanent (red?) Loctite on threads broke after a few dozen rounds; might still be worth a try in an AR barrel nut... But honestly; he calculated recoil to be some 500g in front to back vibration (gravity units, not grams; all his measurements are imperial units); that's some serious "impact" on an aluminum upper receiver.

I trimmed up the few 300 Blackout test cases I had left to max length, + 0.002 for the time being. All cases measured +/-0.001 in height between different rotations of the case, and the whole lot. These first few cases will be used to test the chambers after casting, and find the bullet seating to set overall length for each projectile I have. Vaughn argues that sizing all bullets is necessary because it can help true the base; also that sorting by weight is necessary as well.

Plenty more to read; but these authors give me a very good set of constraints already. Apparently years of testing went into Vaughn's book; and it's all sound science. He was clearly a master in many fields, and difficult to ignore on these topics. 

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On 3/8/2019 at 8:40 PM, Lane said:

Apparently I only have two options (so far); but all of these powders have QuickLOAD data of course. Does anyone know if the IMR and H 4198 are technically different? I see different grain weights for them; should I use both and call that three? I probably need to get a few off the partial data list as well in terms of burn rate comparison.

Crosses between 7.62x39/300 Blackout well: Accurate 1680, Both 4198s?

Partial data across one or both cartridgesBL-C(2), H110, Win 296, Benchmark, IMR 4227.
 

IMR 4227 is what I use for my .30 Carbine loading, and it's a good powder - I've seen many references for it for 300BLK.  H110 and Accurate 1680 I have loaded myself for 300BLK, and have both on-hand for it.  I like the 1680 for the 150gr FMJ-BT loading, works well.  The H110 did well at that, too, but got hard to find for a little while, and I found the 1680 instead. 

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14 minutes ago, 98Z5V said:

IMR 4227 is what I use for my .30 Carbine loading, and it's a good powder

Vaughn talks about IMR powder in some depth. His understanding is all IMR powders (at that time?) are very similar, single base powder; and mostly differ in grain size. He has a nice table; and 4198 is the smallest at 0.026". He didn't list 4227, so if you ever get a chance to measure some; that would be cool. 

I found some redneck loads of Lil Gun in 7.62x39, so that might make the list. Someone mentioned it was a shotgun powder? Slow as pistol powders go? I can certainly work on my own data in the burn rate department... 

Looks like RIT dye, or RIT synthetics dye is used to color polymer80 and Magpul products. These will still be black rifles; but this particular one will be purple on the inside.

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Barrel blanks are at the fabricator as of last Friday, and I got yet another thank you letter from Green Mountain Rifle Barrel Co. along with a sticker. They send it in the mail separately; but a truly nice touch in my opinion. Not to mention the fact that they drop shipped the barrels to Compass Lake Engineering for me, and still had the presence of mind to send the thank you letter to my house. Cerrosafe showed up in the mail today; chamber casting should be fun when that time comes...

 

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I read up on the @SimonSays inspired radio chronograph this morning while I was trying to fall asleep. While I have not done any physical testing yet; the concept does have legs. It will require a minor modification to the circuit interface, appropriate RF blocking (something like a horse blinder) for each unit, and some sort of analog interface. I haven't decided if this needs to be a microcontroller build, or something more old school to count the time between "gates". My only concerns left are about how well it will detect small projectiles (like .22lr), how far away from the sensors one can shoot, and the amplitude as well as width of the disturbance to the radio circuit is in time. 

Good minds have looked this particular sensor over and offered strong proof that doppler radar is not involved. I also didn't realize it was 3.2GHz; which is well in the realm of watching with a software defined radio for testing. When I get to that point, a slug of metal (perhaps half the size of a .22lr) on a wooden dowel rod should give me a reasonable look at how far away the sensor can detect projectiles. 

Got a call from Compass Lake Engineering today. I was quite sure that was going to happen when the fabricator saw my specs. His comment at the end was, "you New Yorker's got it bad". I didn't offer up too much more information other than an explanation of the 21.750"; and that this was really for fun. Sounds like they will be loaded up for fabrication today... So I'm going to need to button up my other work around here very quickly...

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Still need a few important parts for these builds; but much of it is already here. Considering options for the strain gauge (or bridge) connectors; 3.5mm stereo might be good choice for the best of both worlds, and mount easily in the handguards. I typically use RTV to keep wires strain relieved in high vibration environments; might want to go with black for concealment in this case.

Found out there is a 1,000 yard range nearby that I drive past on a regular basis; only there are strict requirements for obtaining membership... There really is no sense building target rifles if I'm not going to compete, this seals the deal on that issue.

Have a box full of clean cases for reloading .308 Win and 7.62x39 in preparation for this 300 Blackout project. I haven't settled on a distance from the lands yet. I see a lot of argument between deep in, and 0.010" back. There is an awful lot I can do with those rifles now in terms of collecting data, and working on the basic methods. There are a few nice days coming up again, and I should be able to hit the local 100 yard range pretty soon.

I have a good feeling these 300 Blackout barrels will be shipping this week, if not arriving. I have hardly prepared for that; but can assemble one right away. The one thing I'm seriously lacking is the upper lapping tool; but I don't see too much harm in doing a barrel test and swap before the initial lapping process. I do want the strain gauges installed for that swap test though; so that is a priority. I have not decided on an accelerometer yet. I found a few more options in piezo around the $150-$200 range; but really want to be sure of my choice. I only plan to buy one, and move it around as needed. That also requires the mount be calibrated with a test weight, since that type of observation will absolutely disturb the barrel resonance significantly. Kolbe goes through those calculations though; and it wouldn't be difficult to estimate either. 

Designed a .223, to 300 blackout conversion jig (for cutting). I'm not sure if I'll be able to do a whole tray of 50 at a time. I'm planning to start with a single row and then expand based on my experience. Haven't had a lot of fun time recently; trying to bank it up though, considering the depth of this project. 

Brief (raw) summary of the hypothesis: The 21.750" barrel length doctrine seems like a stretch given the amount of other research surrounding the Houston Warehouse experiments. On the other hand; there may actually be some breakpoint in barrel resonance that confirms a similar length based on excessive feedback. It is entirely possible that a barrel much longer that 21.750" will feed back too much undesirable resonance. It is equally reasonable to believe that shorter barrels would have a significant deficiency in terms of accuracy. While I am unable to recreate, or test those exact guns; I plan to explore OBT theory against the 21.750" doctrine; while measuring the resonance with more modern technology. By swapping barrels, uppers, and lowers in the AR platform, I can tune external and coupled resonance, while monitoring those changes at the end of the barrel. All existing research points to the same tuning of barrel position in motion to bullet exit time (really in X, Y, & Z); but all sources generalize this to vertical motion in terms of accuracy. This aims for the exact same outcome that ladder load testing a tightly specified cartridge overall length will return (with good neck sizing of course); but with vibration and chamber pressure monitoring added for comparison to OBT theory.

Process (raw unfinished): Establish a testing environment with repeatability, and data capture abilities (.308/7.62x39). Cut a 24" 300 AAC barrel while monitoring results vs. the reference 21.750" barrel build. Swap components to determine effect on resonance to whole system. Cut down below 21.750" to search for a decrease in accuracy that grows with further barrel length reduction.

The Shot Timer will be employed for every one of the tests as a matter of course. I am aware of a timer rollover bug around the 72 minute mark that I need to address. I also need to work out the absolute accuracy of the timing loop to determine how accurate the recorded data is. I may need to round up, or add another digit as development winds down. 

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Stopped at the local shop to grab some powder that was on my cross referenced list; not a single one of those choices were in stock. I don't even recall seeing more than one that worked for either 300 Blackout or 7.62x39. Midway has a free Hazmat sale; but I'm not quite to the point that I want 6-8 lbs. Too many other things on the list before I get to that point in terms of need. I have a powder that will work just fine for empirical observation; and simulation can obviously come later when I need to investigate Long's formula.

I ordered a few other doppler radar sensors last week to round out the @SimonSays radio chronograph testing and builds. About a dozen units total across 24GHz, 10GHz; and the very cheap 3.2GHz (not doppler) for modification.

Writing up a rough hypothesis was quite helpful in getting me to think through what I need to prepare for. It also helped remind me just how many different tests will need to be performed in what order. I've since come up with a number of other points that I need to work into that hypothesis. I've decided to pose a string of questions which will help me craft the methodology first; and then backtrack making suppositions about the outcome of each of those tests to craft the final hypothesis. I've added citations that I could remember off hand; there may be more cross citations than I recall at the moment. Much of the research I've read to date agrees strongly across many different points; these are the questions I still have left after the dust has settled.

Does the upper and/or lower receiver in an AR significantly change the resonance at the muzzle end of the barrel (Mallock)? Does a handguard appreciably change that resonance alone? Can adding pressure between the handguard and bottom face of the barrel damp resonance effectively; or does it reduce accuracy by negating the free floating barrel design (Vaughn)? Does OBT effectively predict the appropriate load for a given barrel length (Long)? Does a barrel longer than 21.750" resonate more strongly (or wildly) than that reference (King et al.)? How full does a cartridge need to be to achieve high performance (accuracy); does it need to be compressed, or simply full (Vaughn, Long, King, etc.)? How close to the lands do bullets need to be seated for accuracy (between 0.010" back, up through press into the lands) (variation among King, Long, Vaughn)? Does tuning to observed OBT benefit from extremely tight measurements of powder charges (Lane)? Could a muzzle end weight be tuned much like a clock pendulum is adjusted to make any load meet OBT (Lane)?

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Circled back over some of Harold Vaughn's book "Rifle Accuracy Facts" which appears to have been scanned by the Army Research Lab in MD, and is available through archive.org. Here is a link to the .pdf with images; it is available in about a dozen various e-book formats if you google it.  https://archive.org/download/RifleAccuracyFactsFullV1.0FirstFullScan_201705/Rifle Accuracy Facts Full v1.0 (First Full Scan)_text.pdf

I was most interested in matching his strain gauge setup, since he too was using an oscilloscope in the field. Turns out I was able to find some dual strain gauges with leads from a surplus supplier that meet all the specifications. That gives me five to install; and it saves me quite a bit of money (while maintaining the same or better accuracy). I also looked over the accelerometer more closely; and found some of those at reasonable prices as well. The trick there was to stay out of the digital offerings which simply aren't fast enough for this kind of work. Should be able to use the same 4 channel scope on the strain gauges and accelerometer(s), and it can even be operated remotely from the firing location here. 

Found some partial answers to some of my questions in terms of powder and bullet seating; but it will still require testing. I'll just keep taking notes here until the barrels arrive and work that into my reloading process. In the mean time; tomorrow is a nice day so I'm planning to load up some new rounds for 7.62x39 and .308 to begin tuning. I have a box of ladder loads for the .308 already, and making some for the 7.62x39 will be a priority tonight or tomorrow morning. The 7.62x39 will probably be the first to get a chamber pressure gauge so that I have some experience under my belt when I install them on the 300 Blackout barrels. It will also be helpful in calibrating based on factory loads I still have for that gun. 

Ordered a crow's foot since the barrel nut torques will be an important metric (and I have to pull everything I have to lap upper receivers). Can't really estimate on torque given all the rest of the complexity here. Only things left to order are loctite 609, more perma blue, and some miscellaneous parts for the second 300 Blackout build. I was planning to do a barrel swap early on; so I haven't even decided which is first vs. second. 

The one set of questions I can't find much information on is this stock contact/coupling issue (annotated below on Vaughn's wired up rifle). Is that going to skew the barrel length calculations for OBT, or the 21.750" theory? Vaughn actually talks about adding pressure to that end before bedding the barrel. That's still possible with a shim between the handguard and barrel in an AR; but that breaks the free floating design too. I really don't know which is more important in this case; and it may not matter at all if I tune with barrel end weight like Kolbe. 

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Two variations of easy to build blinders for the @SimonSays inspired radio chrono build. The reason for two designs is that I'm not sure how much clearance I will need around the circuit. The specs state a minimum of 1cm; but I may be better off with an inch or more. I might as well do some testing if I'm going to be outside. I should be able to watch it with a handheld SDR without having anything more than power wired to the units. It will also be interesting to see how close in frequency a pair of them operates. 

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