Lane's: jtallen83 inspired 21.750" 300 AAC Blackouts

[ARTrooper]

1 hour ago, Lane said:

Sure enough; I saw a bunch of 1 pound exploding targets at the gun shop today. There wasn't a price on the shelf, but when I got home I looked it up. They aren't very expensive, I should have grabbed a handful. Next time for sure...

The stuff is pretty cheap and a lot of fun. makes a bigger boom than one would expect. 

my neighbor shot one pound once and I went over there thinking he blew his house up. lol. they expected their neighbor cop to show up after such a big bang. lol.

[Lane]

21 hours ago, 98Z5V said:

You will be much better served with a 1:8" twist over a 1:7" twist, if you end up with that 78gr projectile.

I THINK I finally have my head wrapped around this; but feel free to add any correction. While the 1:7 twist is considered the standard that covers the whole range of 300 Blackout; if I'm trying to key in on the supersonic range, I may want something different.

I did some more shopping around for future barrels in the process. Green Mountain sells this particular 1:7 blank specifically for 300 Blackout, but I should have considered their other blank options in .308 caliber. Those are available in 1:8, 1:10 and 1:12 twists. Fabrication won't know the difference or care; they are all .308 barrels...

These initial 1:7 barrels can still give me an idea roughly what twist I'm going to want in a light projectile / supersonic optimized barrel (which may not even work for the heavy projectiles if taken to the extreme). The basic Greenhill formula I found doesn't account for bullet speed (very well); but can give me some estimates none the less. T=150(d/r): T=150(.308/(1.150/.308))= a twist of 1:12.3735 for a 165 grain Speer Hot-Core. Now; that 150 in the formula is apparently a constant related to bullet speed (I just don't know how to adjust it accurately). With some bracket testing I suspect the reality for the 300 Blackout may be closer to 1:10 or even 1:8 with the roughly 2000 FPS expected in a long barrel.

I had a box of 155 grain match grade projectiles in my hand earlier; but nobody in the shop could find load data for 300 Blackout in 155. Turns out Lee had it, but not with CFE-BLK. Sierra actually provides the same load data for their 150 and 155 grain projectiles;  I never thought to look at the projectile manufacturer's site before. 

Not sure I totally understand why lighter is going to be more accurate, but that's a problem with my incomplete knowledge on the subject. I'll try to work that out in my head as I do some load testing.

[98Z5V]

1 hour ago, Lane said:

Not sure I totally understand why lighter is going to be more accurate, but that's a problem with my incomplete knowledge on the subject. I'll try to work that out in my head as I do some load testing.

Muzzle velocity.  The most I can drive my 300BLK 150s to is about 1800fps through a 16" barrel.  It's not even enough to explode Tannerite at 100 yards.  Tried it many a time, thinking that I was missing the target - only to walk down range and find that I gave the 2-liter bottle a huge leak.  Went back to the firing line and shot it with the Mk12 and BOOM!...

Inherent accuracy in a projectile is in the bullet construction and muzzle velocity - how many people you know can shoot MOA with a 5" 1911 at 50 yards?   The projectile isn't designed for that, it's subsonic, fat, slow, and that was never what it was meant for.  Muzzle velocity is your friend in accuracy testing, as long as you hit an accuracy node in the loading, and stop there, before pressure signs.

You picked a real tough cartridge to pull for an accuracy-over-barrel length test, that's for sure.  I don't doubt in the least that this is gonna be good, and we're all going to find out alot.  But you need speed, man.  Heavies and barrels designed with fast twists for heavies aren't going to be the answer, overall.  That 1:7" might not be bad at all, but the 1:8" os the "overall" twist on BLK - handles all of them about as well as a twist rate can, from superlight to 220 subs.  That 1:7" is more for the higher end of the weight class on these, but it still might do great with a 125gr or something.  

This will be good, that's for sure.  

 

[Lane]

Ordered the tube micrometer, and case length gauges for most everything I am reloading now. The trimming die should be here tomorrow for 300 AAC Blackout; I'll cut everything I have (a bit long) when I see it. I have a bunch of other cartridges to trim when I do; might as well collect all the brass shavings in the same sitting.

I picked up some crude tools to cut brass down for 300 Blackout (and more once fired 5.56 brass). I was at harbor freight a few times recently, but didn't buy much; certainly not that chop saw at $35+tax. I wanted to, really; but it seems like I have better options around here already. Never going to use a tubing cutter on cases again, but it does work fine with a well adjusted tool. I just don't see the point when I'm going to trim it in the die anyway. That end can be raw and nasty. If I do it right I might get an ok anneal when cutting cases en masse.

[Lane]

1 minute ago, 98Z5V said:

only to walk down range and find that I gave the 2-liter bottle a huge leak.

Lol. Yes. I will be shooting explosives when I KNOW I have 2001 FPS down range. I'm going to have to invite my neighbor when I do; not because he would be scared, but he'll want a few targets too. When the 21.750" fires straight and fast; that will be my first task. 

4 minutes ago, 98Z5V said:

That 1:7" is more for the higher end of the weight class on these, but it still might do great with a 125gr or something.  

This was really a comparison to the 7.62x39 gun with a 21.500" barrel. I should be able to shoot the 21.750" 300 AAC side by side, and get some good reference data. And I wanted that gun to be purposeful. I think I could hut the 21.750" if 150 grains are the one... Empirical data is the only way I'll be convinced on that front.

6 minutes ago, 98Z5V said:

This will be

This will be fun. And I can't see past my forth barrel in this project yet. The third will be ordered with the .260 barrel to save on shipping again. Armalite uppers should be available quite soon... I'm so excited.

[Lane]

Which forge marks are on your AR-15 uppers? I would like some input here.

I need another upper or two; and I think that is a worthwhile investigation. Houston Warehouse article talks about resonance; and that matters. If barrel nuts are mated with upper receivers, I should be able to fool around with barrels a bit in this way.

I'm a firm believer in always having; but never using the forward assist. Might need to try a few other options though; to see if that make a difference. 

[98Z5V]

1 hour ago, Lane said:

Which forge marks are on your AR-15 uppers? I would like some input here.

 

All kinds, but I can tell you that forge marks don't really matter.

[Lane]

2 hours ago, 98Z5V said:

All kinds, but I can tell you that forge marks don't really matter.

I want to believe this; I really want to drink that Kool Aid... But there is a point in this experiment that needs to be proven otherwise. My efforts may never make that dent; but I still wish to know. In a free floating barrel design; the upper is the only other variable (given appropriate lower constraints).

[jtallen83]

There was an article posted here recently about AR accuracy that mentioned a difference in upper strength making a difference in accuracy but it didn't call out any particular brand. Just said some are machining away too much material for lightness. I doubt you could go wrong with Armalite uppers. 

[SimonSays]

On 2/27/2019 at 8:28 PM, Lane said:

 The research suggests that the powder charge doesn't even matter; simply the barrel length, and cartridge/bullet to barrel tuning. There is a supposition of resonance, but I fully intend to refute that during my exploration and analysis of this process.

I never understood how resonance alone could be responsible for shot dispersion. Take 175 gr FGMM for example. It's shot in so many different 308 guns and preforms remarkably in all of them. I ran across a theory developed by Christopher Long that I think nails the explanation. If his theory is correct, I'm not sure one could ever prove what barrel length is optimum.   http://www.the-long-family.com/shock wave theory summary explanation.pdf

[Lane]

6 hours ago, jtallen83 said:

difference in upper strength making a difference in accuracy

I'll take this as a vote for "heavier may be better". My Bear Creek side charging upper is a totally different profile, and noticeably thicker in some places. I'll see what other options I can find in terms of weight difference.

1 hour ago, SimonSays said:

If his theory is correct, I'm not sure one could ever prove what barrel length is optimum.

This was my initial thought on the barrel length discussion as well. But I won't be totally convinced until I see the data side by side; chopping a 24" barrel next to the 21.750" reference build. As has been earlier discussed; I don't expect my small sample size will amount to proven fact, but it will help me form an opinion for myself.

And; I stopped at the store while I was out taking care of other business today... I might not be able to save them all for these builds; if it weren't snowing I might be outside right now trying to break rocks.

[jtallen83]

10 minutes ago, Lane said:

I'll take this as a vote for "heavier may be better".

That was the jist of the articles statement. I think this is the article I remember reading;

http://bulletin.accurateshooter.com/2015/05/what-makes-an-ar-accurate-whitley-offers-answers/

[Lane]

1 hour ago, jtallen83 said:

http://bulletin.accurateshooter.com/2015/05/what-makes-an-ar-accurate-whitley-offers-answers/

That was a good read. Sure enough; there is a thick walled, side charging upper in the pictures. They also recommend Loctite on the barrel as a substitute for real bedding in the upper receiver; something I've been contemplating as well. 

Everything else checks out except for truing the receiver... That is something I don't feel I have accurate enough tools for at home. I may have to find a machine shop that wants to do that if becomes a real concern. I suppose that won't be possible to determine without a surface plate, so add that to the list of tools outstanding for this project. The cheap stuff is flatter than 0.0001" which should be ok, but I'll see what I can afford. It doesn't need to be huge for this application. 

[jtallen83]

1 minute ago, Lane said:

Everything else checks out except for truing the receiver...

Hard to go wrong with this lapping tool. I have used both the AR-15 and the big brother with good results, required nothing more than my cordless drill, upper receiver vise blocks, and vise. Used the lapping compound from my scope ring lapping kit. They are limited in usable life depending on how rough the receivers you lap are.

http://pacifictoolandgauge.com/ar-tools/1139-ar-15-upper-receiver-lapping-tool.html

9 minutes ago, Lane said:

They also recommend Loctite on the barrel as a substitute for real bedding in the upper receiver; something I've been contemplating as well. 

Loctite 609 for mating the extension and the receiver.

https://tdsna.henkel.com/NA/UT/HNAUTTDS.nsf/web/3E906D6B842166B0882571870000D855/$File/609-EN.pdf

 

[Lane]

21 minutes ago, jtallen83 said:

Hard to go wrong with this lapping tool.

Cheaper than a surface plate, and does the job I need; check. Now that 6-8 weeks for fabrication business is a problem. Found a Wheeler at Midway, and Brownell's sells something for a few dollars more. Not really fond of the look (quality?) of the Wheeler; and the Brownell's products looks like the Pacific Tool product. Seems like an easy decision considering I have a few uppers for this project. 

34 minutes ago, jtallen83 said:

Loctite 609 for mating the extension and the receiver.

Same deal here; the Boot_Scraper thread sold me on that for sure, there will be 609 on each build (after I get done swapping barrels around once). 

[Lane]

8 hours ago, SimonSays said:

http://www.the-long-family.com/shock wave theory summary explanation.pdf

Holy crap, I finally got a chance to read this carefully; twice... That was technical; and may well save me a lot of time. It appears most all of the data came from simulation, but proved the best groups in practice too. Apparently, there is an idea exit time based on vibrations in the barrel. If you miss one; you can probably still hit a different one with a different bullet weight/charge combo.

But; you are also correct in that this supposes the barrel length will not matter... An ideal exit time can be hit for any barrel length by matching to a vibration peak.

I would like to replicate some of these simulations with real vibration sensing. Seems like it would be trivial to determine an accurate load for a barrel with that data in hand. The only question in my mind is whether I hand tune loads first, and compare; or calculate first and try to hand tune it afterward.

[Lane]

This Chris Long character is a real powerhouse. It appears he pissed all over the 21.750" doctrine more than 15 years ago. That assertion of resonance in the Houston Warehouse tome appears to be true; but not at all related to the 21.750" barrel length. 

He has done some further research and circles back to say this: "OBT theory clearly supports the prediction of the best time to leave the barrel so that the Jump (incidental yaw from barrel muzzle change in shape and rotational angle due to the shock wave strains) is minimized. The POI is still determined by the main vibration modes of the barrel, but the dispersion is all Jump. Minimize Jump, and the Swerve component is also minimized (assuming that the bullets are of high quality)."

From what I can tell, he never did go back and measure the vibrations with strain gauges and acoustic pickups. His formula appears to be so airtight it might not even matter; but it's not much of an investment to collect the physical data myself. 

This appears to be a home run; and something I can easily test with any of my other builds right now (minus the 21.750" comparison).

[Lane]

I've got some other things I really need to work on; but I just want to leave this here in the mean time... I'm aware my colored annotations might actually be off by some vertical shift; this is still a general observation.

No longer totally convinced this is the final word on the resonance issue. I think this basic graph ignores a second dimension (actually both X and Y); it appears to only look at Z (or girth). Regardless of how that was devised; there is clear support for this idea based on observed stringing when load testing. Once you cross a node at one of these points bullets will go either vertical or horizontal. Chris's empirical data actually shows the possibility of another node where he couldn't tune the load near 1.188 ms. It is my belief that there are a number of other points to test; one of which might be, at the very least; similarly accurate in terms of group size.

His research very much makes a case for 1 FPS load tuning in my opinion. To hit these minimum jump points; a bullet needs to exit the barrel in a very precise window of time.

[98Z5V]

43 minutes ago, Lane said:

His research very much makes a case for 1 FPS load tuning in my opinion. To hit these minimum jump points; a bullet needs to exit the barrel in a very precise window of time.

 

I'll admit I haven't read that link yet - but when I go to it, I might realize that I've read it before.  I've done some time looking up OBT.

However, my question on it (without reading it) is this: Was that bolt gun data, or gas gun data?  With gas guns, you have a whole new world that you need to account for, with dwell time being one of the biggest factors.  Barrel profile, they're all right-hand twist, so you can account for that in the harmonics (big part of the harmonics), those things are secondary, but important, but dwell time it the one on gas guns.

Quote

Apparently, there is an idea exit time based on vibrations in the barrel. If you miss one; you can probably still hit a different one with a different bullet weight/charge combo.

True, but you also have to calculate that timing with a different bullet and load - to an accuracy node in that bullet and load.  The OBT info accounts for this, but you need to test it on that specific load that the OBT states is accurate - and it's on.  But test it. Gas guns complicate this, with dwell time...

Edited March 7, 2019 by 98Z5V

[SimonSays]

2 hours ago, Lane said:

Holy crap, I finally got a chance to read this carefully; twice... That was technical; and may well save me a lot of time. It appears most all of the data came from simulation, but proved the best groups in practice too. Apparently, there is an idea exit time based on vibrations in the barrel. If you miss one; you can probably still hit a different one with a different bullet weight/charge combo.

But; you are also correct in that this supposes the barrel length will not matter... An ideal exit time can be hit for any barrel length by matching to a vibration peak.

I would like to replicate some of these simulations with real vibration sensing. Seems like it would be trivial to determine an accurate load for a barrel with that data in hand. The only question in my mind is whether I hand tune loads first, and compare; or calculate first and try to hand tune it afterward.

That was my hope when I posted the link. I can’t remember if you ever stated that you had the Quick Load program he referenced. I have it and would be happy to crunch numbers for you. 

[Lane]

35 minutes ago, 98Z5V said:

But test it. Gas guns complicate this, with dwell time...

Looks like he was testing bolt guns. These 300 AAC Blackout builds have no gas ports drilled, for this reason; among others. Also; couldn't find a gas block to match barrel diameter, didn't want to interrupt the barrel profile, and the gas system makes a mess where I don't want it. In general; I assumed making these semi-automatic would reduce achievable accuracy.

36 minutes ago, SimonSays said:

That was my hope when I posted the link. I can’t remember if you ever stated that you had the Quick Load program he referenced. I have it and would be happy to crunch numbers for you. 

I do not have Quick Load (yet); but he provides the formula to return each of those nodes at the bottom of the paper; under the heading Master Model. A, B, C, and D are all constants; L is barrel length in inches, and N the node number: OBT in ms= (A*N + B)*L + C*N + D. I think 98Z5V just stated that I will need the software to get the data for my specific loads though. This is still a huge step forward in terms of exploring resonance theory with authority. I can do all the calculations necessary to meet these specifications; then try to search for any missing nodes in the gap experimentally. At the very least it will be interesting to see how the groups evolve across that resonance profile.

Looks like he swears by hardware called PressureTrace, which is little more than a strain gauge attached to the barrel; and an oscilloscope. That type of sensor hardware was already on my shopping list for this project; and I have an oscilloscope behind the wall I shoot next to here at home. The PressureTrace site has some interesting graphs about what a good load looks like; and those secondary peaks on BAD loads are pretty scary. I suspect the tiny necked 300 Blackout factory ammo I have here might be able to do just that; but I don't want to see it happen in these builds if I can help it.

QuickLOAD software is only $150; clearly it's going to save me a lot more than $150 in aggravation trying to reinvent the wheel in this case. Adding it to the list right now...

[Lane]

In case anyone cares; this guy attached a strain gauge to measure chamber pressure for less than $10. Clearly my kind of solution...

http://www.ktgunsmith.com/straingauge.htm

[SimonSays]

19 minutes ago, Lane said:

In case anyone cares; this guy attached a strain gauge to measure chamber pressure for less than $10. Clearly my kind of solution...

http://www.ktgunsmith.com/straingauge.htm

The software is only $100. That’s a really nice setup. You are going to have so much fun. 

[SimonSays]

I got the Labradar for Christmas. With that being able to intergrate with the software... and the gauges so cheep... maybe ???

Nope. I really need to get a new scope and bipod first. But, I will put this on my to get list. 

[Lane]

9 minutes ago, SimonSays said:

You are going to have so much fun. 

That is the truth sir. 

2 minutes ago, SimonSays said:

Labradar

 I haven't settled on a chrono solution; can't hurt much to use more than one. I would be curious to know how tight you think the measurement data is. Does it meaningfully measure 0.1 fps? The specifications on the Labradar are a bit loose in terms of accuracy; just states "better than"... Everything in the optical chrono department is laughable in terms of accuracy.