Cycling issues -- 20" Ballistic Advantage 308 barrel, rifle length gas (high frame rate video included)

[renaissanceman]

I'm new to the AR10 world, and I'm having issues with cycling.  Bear with me -- there's been a progression of testing on this issue that will be covered in detail  (TLDR: skip to video (https://streamable.com/3idj1i) and comment on undergassed symptoms)

Parts in build:

-Aero upper
-20" ballistic advantage barrel, rifle length gas/0.086" port with a Kaw Valley Magnum linear comp, (no suppressor)
-Standard BCG (unbranded, from Right to Bear)
-Gorilla Machining adjustable gas block with Expo arms rifle length tube
-Expo ar10 charging handle
-PSA lower parts kit (carbine buffer tube, 2.8oz short buffer, ar10 buffer spring)
-Magpul 10 round mags
-Running wet with ALG go juice.

 

The bolt can be pulled back by hand and locked on an empty mag.  There's about 3/8 of an inch of travel past the stop.  The gas tube sits at the halfway point in the cam "half moon" so I don't think that is the issue here.

I was shooting 147gr handloads with shooters world tactical rifle powder, all the way from min loads worked up to max, and there is no difference.  I first assumed undergassed since I got no lockback on a single round, but from some reading and advice from those on AR15.com, I learned people generally think that AR10s that do not run are are usually overgassed.   After reading that, I installed a Gorilla adjustable gas block, and started with it closed and slowly opened it while firing single shots of a mid-range load.  It takes 6 turns from full closed to full open, and at 1 turn, the bolt unlocked, but the casing didn't move from the chamber.  At 2, it pulled about halfway out.  At turn 3, the casings began to eject and fall right below the rifle.  Turn 4, 5 and 6 resulted in them landing farther and farther away, with the shells landing pretty consistently about 5 feet back at the 4 o'clock position at full open, similar to before the adjustable was installed.  I did not get a single lockback on about 10 shots fired with the block full open.  

At this point, I'm back to thinking it's undergassed.  Quickload shows port pressured in the 14.5ksi range, whereas most 223 loads show port pressures around 20ksi.  The 0.086" port is similar to, if not smaller than, what I've seen in AR-15s, and it seems a touch small for a 14.5ksi port pressure.  Tactical Rifle is a bit on the faster end for 308 powders, so I'm going to try loading up some 168gr BTHPs over Match Rifle, which is a lot slower.  I'm thinking I can get port pressures up to 16ksi or higher with that combination.  If that doesn't result in lockback, I'm considering drilling to around 0.089 or 0.0935" and trying again.  I've had to a drill port in a big bore AR15 barrel out in the past (357 AR -- 8-12ksi port pressures at carbine length), which fixed short stroking, but I'm not familiar with the quirks of the AR10, other than I've never seen one that just runs without issues that need to be worked through.  

Later, I shot with the same 147gr handloads, tula of a couple lots, and 168gr bthp loads to see if I could get bolt lockback.  I couldn't and I captured 960fps video of all tests.   That footage can be viewed here:

https://streamable.com/3idj1i

Thoughts?  

[Matt.Cross]

3 minutes ago, renaissanceman said:

I'm new to the AR10 world, and I'm having issues with cycling.  Bear with me -- there's been a progression of testing on this issue that will be covered in detail  (TLDR: skip to video (https://streamable.com/3idj1i) and comment on undergassed symptoms)

Parts in build:

-Aero upper
-20" ballistic advantage barrel, rifle length gas/0.086" port with a Kaw Valley Magnum linear comp, (no suppressor)
-Standard BCG (unbranded, from Right to Bear)
-Gorilla Machining adjustable gas block with Expo arms rifle length tube
-Expo ar10 charging handle
-PSA lower parts kit (carbine buffer tube, 2.8oz short buffer, ar10 buffer spring)
-Magpul 10 round mags
-Running wet with ALG go juice.

 

The bolt can be pulled back by hand and locked on an empty mag.  There's about 3/8 of an inch of travel past the stop.  The gas tube sits at the halfway point in the cam "half moon" so I don't think that is the issue here.

I was shooting 147gr handloads with shooters world tactical rifle powder, all the way from min loads worked up to max, and there is no difference.  I first assumed undergassed since I got no lockback on a single round, but from some reading and advice from those on AR15.com, I learned people generally think that AR10s that do not run are are usually overgassed.   After reading that, I installed a Gorilla adjustable gas block, and started with it closed and slowly opened it while firing single shots of a mid-range load.  It takes 6 turns from full closed to full open, and at 1 turn, the bolt unlocked, but the casing didn't move from the chamber.  At 2, it pulled about halfway out.  At turn 3, the casings began to eject and fall right below the rifle.  Turn 4, 5 and 6 resulted in them landing farther and farther away, with the shells landing pretty consistently about 5 feet back at the 4 o'clock position at full open, similar to before the adjustable was installed.  I did not get a single lockback on about 10 shots fired with the block full open.  

At this point, I'm back to thinking it's undergassed.  Quickload shows port pressured in the 14.5ksi range, whereas most 223 loads show port pressures around 20ksi.  The 0.086" port is similar to, if not smaller than, what I've seen in AR-15s, and it seems a touch small for a 14.5ksi port pressure.  Tactical Rifle is a bit on the faster end for 308 powders, so I'm going to try loading up some 168gr BTHPs over Match Rifle, which is a lot slower.  I'm thinking I can get port pressures up to 16ksi or higher with that combination.  If that doesn't result in lockback, I'm considering drilling to around 0.089 or 0.0935" and trying again.  I've had to a drill port in a big bore AR15 barrel out in the past (357 AR -- 8-12ksi port pressures at carbine length), which fixed short stroking, but I'm not familiar with the quirks of the AR10, other than I've never seen one that just runs without issues that need to be worked through.  

Later, I shot with the same 147gr handloads, tula of a couple lots, and 168gr bthp loads to see if I could get bolt lockback.  I couldn't and I captured 960fps video of all tests.   That footage can be viewed here:

https://streamable.com/3idj1i

Thoughts?  

Your recoil buffer system isn't up to snuff, PSA has yet to get it right.

Post up the internal depth of the buffer tube, preferably with pictures.

[renaissanceman]

1 hour ago, Matt.Cross said:

Your recoil buffer system isn't up to snuff, PSA has yet to get it right.

Post up the internal depth of the buffer tube, preferably with pictures.

I'll do that.  It's a standard AR buffer tube if I recall correctly.  

[edgecrusher]

3 hours ago, renaissanceman said:

I'll do that.  It's a standard AR buffer tube if I recall correctly.  

You’re buffer is half the weight it should be.

[sagebrush]

is your lower a psa, maybe i missed where you said brand of lower. if it is a psa lower it usually doesn't work right paired with any other brand of upper

[renaissanceman]

It's an AMT, DPMS pattern.  It doesn't seem to be causing any issues.

[renaissanceman]

52 minutes ago, edgecrusher said:

You’re buffer is half the weight it should be.

How would this explain the bolt under travelling, though?  Seems a heavier buffer would make my symptoms worse?

[shooterrex]

It has to be balanced. Gas system and recoil system. 5.5 oz is about the standard carbine type buffer for the 308's. The light ones don't allow the bolt to slow down enough for things to work properly.

[renaissanceman]

21 minutes ago, shooterrex said:

It has to be balanced. Gas system and recoil system. 5.5 oz is about the standard carbine type buffer for the 308's. The light ones don't allow the bolt to slow down enough for things to work properly.

Yes, all the factors must be balanced and please correct me if I'm wrong here, but if the gas supply is held constant, doesn't adding more weight to generally slow the bolt speed and thus compensates for  an overgassed condition?

Edited November 21, 2020 by renaissanceman

[DNP]

4 hours ago, renaissanceman said:

I'll do that.  It's a standard AR buffer tube if I recall correctly.  

Define standard AR buffer tube. 

[Matt.Cross]

51 minutes ago, renaissanceman said:

Yes, all the factors must be balanced and please correct me if I'm wrong here, but if the gas supply is held constant, doesn't adding more weight to generally slow the bolt speed and thus compensates for  an overgassed condition?

You're up against at least two factors, if not three, all of which stem from PSA sucking at design and going for cost savings in manufacturing. Your recoil buffer and spring are the known bad factors, the only question remaining is whether or not your buffer tube will work with the correct buffer and spring. Once we have the internal depth of your buffer tube, we can tell you whether or not you need to replace all three items, or just two. This is why we want measurements with pictures.

Explaining the physics isn't productive when a buffer tube that's too short will never allow adequate function to begin with....

[98Z5V]

Your recoil system is fucked.  I'll say it.  Read this, first, then respond after you've read it.  Ask any questions about the information,  right here in your thread - NOT in that thread.

Read:

EDIT - P.S.  5.4 ounces is the standard buffer weight.

Edited November 21, 2020 by 98Z5V

[renaissanceman]

38 minutes ago, 98Z5V said:

Your recoil system is fucked.  I'll say it.  Read this, first, then respond after you've read it. 

I like a man who tells it as it is.  I'm getting the picture -- I'll read the thread you linked and then measure when I can.  The rifle lives at the farm, not my house, so it's a drive away to go check or I would have already done it.  

The Palmetto kit uses an AR-15 buffer tube and some janky shortened buffer to make it work.  I noticed the spring looked like crap.  

[98Z5V]

2 minutes ago, renaissanceman said:

I like a man who tells it as it is.  I'm getting the picture -- I'll read the thread you linked and then measure when I can.  The rifle lives at the farm, not my house, so it's a drive away to go check or I would have already done it.  

The Palmetto kit uses an AR-15 buffer tube and some janky shortened buffer to make it work.  I noticed the spring looked like crap.  

I appreciate your understanding, and acceptance to read, more than you know.  THANK YOU for looking at the information presented, because it's about a decade of work, and experience, and trial-and-error on these platforms.  It's proven information.

Too many gloss over it, though, and try to argue it, without even looking at it in the first place. I commend you, and appreciate you.   For YOUR time... 

[renaissanceman]

Ok -- read it in full -- makes sense.  I'll start digging into what parts need to be replaced, hopefully tomorrow.  

Palmetto has been decent for AR-15 kits, everything always ran right out of the gate.  This was not intended to be a buget build at all, but I was a bit confused as to what would work and what wouldn't so I ordered a kit.  The kit has been a joke, though.  Takedown pins were wrong, the bolt catch had a pin rather than a threaded pin, etc, etc.  at least the buffer tube and magpul stock might be useful on a 15 at some point...

I've never actually seen an AR10 that runs, but against my better judgement I was suckered in.  I figured there's got to be a way to get them going, and I'd figure it out.  I appreciate the time and expertise you guys freely offer.  

[98Z5V]

Please ask questions, along the way - we'll answer. If something doesn't make sense, just ask.  If something doesn't jive - we'll help, and correct our information if need be.

But, asking is the most important part. At one point or another, all of us here have been through all of the questions that you have - not a single person, but the collective.  We've all done this for a long time, and someone, here, has experience with what you're asking.  We're here to make functional .308AR rifles - no matter what crazy caliber someone dreams up to shoot from them.  We pretty much have all the calibers covered, amongst us, and tinkered and figured them out.   Been a long, crazy road...

If you want it solved - we've pretty much done it already, and we're here to help. 

[renaissanceman]

I measured, and the internal depth of the buffer tube is 7" (measured by pencil... the tape wouldn't fit...), the buffer is 2.5", and the buffer and spring combo is just shy of 12 inches when removed and the spring is at free length (uncompressed). 

 

Where do I go from here?

[shrade]

Here is where I would go: heaveybuffers.com    look at the Car-10 buffer and spring combo. Did wonders for my DPMS Recon 2. It had a 7" buffer tube with a 2.5" buffer. 

[Matt.Cross]

26 minutes ago, renaissanceman said:

I measured, and the internal depth of the buffer tube is 7" (measured by pencil... the tape wouldn't fit...), the buffer is 2.5", and the buffer and spring combo is just shy of 12 inches when removed and the spring is at free length (uncompressed). 

 

Where do I go from here?

Go here: http://heavybuffers.com/ar10carbine.html

and order the buffer + spring combo of your choice, either of the first two weights on that page should work. The "extended carbine" option requires a buffer tube that's 7.625" (7 & 5/8") internal depth, which can also be bought from the same site if you wish.

[shrade]

renaissanceman: Just re-read your original post and noticed you are running P-Mags. My Recon would manually hold open on an empty P-mag, the follower would raise the bolt stop only a fraction of an inch. But would not hold open after firing last round. My theory was the bolt, with too light of a buffer and spring was moving at such a high rate of speed it would ride over the slightly raised stop. I Knew nothing about heavier buffers and springs at the time, so I solved the problem by running Lancer mags which have a raised "lug" on the follower which raised the stop to full height and stopped the bolt. We have to remember there is no Mil-Spec for the 308 platform, so different manufacturers have different  tolerances for their sometimes proprietary parts. I had gotten rid of all my P-Mags prior to upgrading to a heavier buffer set-up, so I cant tell you if that would have fixed the p-mag issue. Hopefully a different buffer and spring combo will fix your problem, If not you may have to go a different magazine rout. 

[renaissanceman]

41 minutes ago, shrade said:

renaissanceman: Just re-read your original post and noticed you are running P-Mags. My Recon would manually hold open on an empty P-mag, the follower would raise the bolt stop only a fraction of an inch. But would not hold open after firing last round. My theory was the bolt, with too light of a buffer and spring was moving at such a high rate of speed it would ride over the slightly raised stop. I Knew nothing about heavier buffers and springs at the time, so I solved the problem by running Lancer mags which have a raised "lug" on the follower which raised the stop to full height and stopped the bolt. We have to remember there is no Mil-Spec for the 308 platform, so different manufacturers have different  tolerances for their sometimes proprietary parts. I had gotten rid of all my P-Mags prior to upgrading to a heavier buffer set-up, so I cant tell you if that would have fixed the p-mag issue. Hopefully a different buffer and spring combo will fix your problem, If not you may have to go a different magazine rout. 

Here's the thing -- I'm not seeing high bolt velocities, and the bolt is getting nowhere near the catch on firing (video shows this pretty clearly). I'm going to correct my buffer issues, but I'm still not sure how the increase in buffer weight will help, unless it helps get the gas impulse to the bolt converted into more kinetic energy to help with the bolt travel rearward against the spring. 

 

My gut is telling me after I get  a new spring, buffer, and tube, I'll still have to drill the port. 

We shall see. 

[DNP]

Likely on the port. It’s closer than we’ve seen, but I agree it could probably go up a hair from what we’ve seen. 

[billymagg]

19 hours ago, 98Z5V said:

Your recoil system is fucked.  I'll say it.  Read this, first, then respond after you've read it.  Ask any questions about the information,  right here in your thread - NOT in that thread.

Read:

EDIT - P.S.  5.4 ounces is the standard buffer weight.

I did read that whole thread again last night for the second time, a gold mine of information and ideas!

 

19 hours ago, 98Z5V said:

I appreciate your understanding, and acceptance to read, more than you know.  THANK YOU for looking at the information presented, because it's about a decade of work, and experience, and trial-and-error on these platforms.  It's proven information.

Too many gloss over it, though, and try to argue it, without even looking at it in the first place. I commend you, and appreciate you.   For YOUR time... 

Its a great read, I finished it again last night, I ordered the 40 coil flat recoil spring for the 300 that Wilson uses in those guns... then I started to wonder what Wilson runs in their 308 AR's, mine runs like a house of fire... just wondering how they do it. The 300 is gonna be a virtual clone of my large frame Wilson. I decided to stick with Wilson, because I know it works!

[renaissanceman]

34 minutes ago, DNP said:

Likely on the port. It’s closer than we’ve seen, but I agree it could probably go up a hair from what we’ve seen. 

Can someone explain to me why with such a light buffer installed, my bolt would under-travel like it does in the footage?  From my experience with AR15s, it should be much too fast if the gassing is sufficient.  I'm tempted to drill the port to 0.093" while waiting on the buffer and spring...

[Matt.Cross]

17 minutes ago, renaissanceman said:

Can someone explain to me why with such a light buffer installed, my bolt would under-travel like it does in the footage?  From my experience with AR15s, it should be much too fast if the gassing is sufficient.  I'm tempted to drill the port to 0.093" while waiting on the buffer and spring...

I wouldn't advise it. You need to keep changes to a minimum, drilling your gas port might cause issues you could've avoided. If the correct buffer and spring fix your issue, you're done. If not, drilling up is the next logical step.