Built a .308 Frankenstein, resulting in confusion

[NateDoge]

Hello,

I recently built a totally piecemeal .308 AR based on a PSA PA10 Gen II lower. I went with the inadvisable path of using parts from many different manufacturers, but I did try to do some research beforehand to make sure that I wasn't going to run into major problems. From what I understood, the PA10 is a DPMS pattern .308 AR, so I simply stuck with that pattern. However, I am now coming to realize that PSA has blended elements of both the DPMS pattern and AR10 pattern in their design (more on that later). For reference, these are the parts I used in my build:

PA10 Gen II Classic Lower

DPMS LR-308 Gen II Upper, low-profile

Ballistic Advantage Modern Series 16" Government Profile .308 Barrel

Toolcraft .308 BCG

Yankee Hill Machine low-profile .750 Gas Block, clamp-on style

11.75" Gas Tube

ALG ACT Trigger

Strike Industries .308 King Comp

I have the whole thing together and have taken it to the range twice. The first time was with an A2 flash hider instead of the comp. I fired 40 rounds with no malfunctions. No problems at all. The second time was after I installed the comp. I fired around 80 rounds, and it short stroked every time. The BCG was not going back far enough to engage the next round in a magazine or lock back on an empty magazine. When cycling by hand, I can easily move the BCG to the proper position and engage the bolt latch or get it to lock on an empty magazine. Ejection was very weak, just a couple of feet away from the ejection port at about 3 o'clock.

Now, my immediate assumption was that the comp had altered the pressure in the gas system and now I am under gassed. I double-checked the alignment of the gas block and gas tube, which did turn out to be ever so slightly off so I re-positioned it. I did many other diagnostic type tests including standing the BCG on the bolt to check the gas rings, making sure that the bolt would slide back and forth in the receiver under gravity at a shallow angle without getting stopped anywhere, etc. I also wrote Ballistic Advantage and confirmed that I have the correct length of gas tube. I do not have many special tools, so I have not been able to measure anything such as the gas port to see if something is out of spec. I did notice that there is a small amount of carbon build up on the gas tube where it enters the gas block, so it appears that some gas is leaking there but it does not seem to be very much. Also, I have cleaned the rifle thoroughly and lubricated it with white lithium grease in between each range session.

Now, here is the part where I got confused. Since I bought a complete PA10 lower, I did not bother replacing any of the parts in it except for the trigger. I assumed that I should leave it all as it was, and that it would be pretty much compatible with other DPMS style parts. Indeed, the DPMS LR-308 upper fits very well.

However, from doing some reading on forums like this, I have come to understand that when people experience short stroking or low-pressure problems, the first things that people advise looking at are obviously the gas system as well as the buffer and buffer spring. This is where I came to realize that PSA has apparently mixed both DPMS pattern and AR10 pattern in their design, because the buffer extension I received is 7.75" according to the AR10 spec. The buffer that I received is a PSA buffer marked "H" and it is the same size as a standard AR15 carbine buffer, so I have to assume it is simply their heavy carbine buffer. According to their website, this buffer weighs only 3.7 ounces, which is on the light side for a .308 as far as I've read. I have no idea what kind of spring is in there but I have written them to find out.

Anyway, it appears that I have put together a rifle that is mostly DPMS pattern except for the recoil system, which is AR10 pattern. Clearly PSA decided that the PA10 should use typical AR15 buffers and possibly springs so I assume that I should take that route when exchanging them.

Based on all of that, I have a few questions:

1) Has anyone ever built a DPMS style rifle with a 7.75" carbine extension using regular AR15 buffers?

2) Which buffers or springs would you guys recommend for use in this kind of a system?

3) Assuming that I am under gassed, and that re-positioning the gas block does not solve my problem, does anyone have any recommendations on fixes? I doubt I will want to go lighter than a 3.7 ounce AR15 buffer.

Thanks in advance for any insight! 

[edgecrusher]

First, the AR10 receiver extension and PROPER spring and buffer will work fine. My guess is the spring is incorrect as is the buffer. Nothing new there from PSA. Your buffer should weigh 5.4 oz. Your spring should be 13.75” relaxed with 34 coils. And for the cherry on top.... your receiver extension is 1/8 too long and PSA is still fucking that up too. Want it fixed for around $50? Buy the complete kit from Armalite.

[ar-mountaineer]

The Armalite kit just came back in stock yesterday. Last I checked they had 14 left 

[NateDoge]

That definitely sounds like a very likely and economical solution. I am 99% sure that the buffer and buffer spring are standard AR15 carbine parts. Looks like I'll be ordering one of those Armalite kits! 

[jtallen83]

46 minutes ago, NateDoge said:

That definitely sounds like a very likely and economical solution. I am 99% sure that the buffer and buffer spring are standard AR15 carbine parts. Looks like I'll be ordering one of those Armalite kits! 

Armalite's AR-10 buffer is an AR-15 H3 buffer, their spring is not, it works in both rifle and carbine extensions, same spring. Buy the kit and you are covered, no issues with it.

[NateDoge]

Thanks for the suggestion, guys. I have one of those kits on the way and I'll let you know the results.

[Ravenworks]

What is with the white lithium grease?

[NateDoge]

I like it because it mostly stays put, lasts through a range session and is designed for high temps. It's also very cheap. 

[NateDoge]

Of course I didn't search for PA10, otherwise I would have come across this thread. Don't buy a complete lower from PSA!

 

[dubya]

10 hours ago, Ravenworks said:

What is with the white lithium grease?

Yeah, I wouldn't put any heavier grease like that in for lube, especially on a new build that needs breaking in. Clean that stuff out and run it wet with a good oil. 

[Ravenworks]

^^pay attention to what he said.^^

Nooks & Crannies

[NateDoge]

I will give that a shot. I've always been in the minimalist camp when it comes to lube but I do oil things up when I'm having problems. When this rifle started short stroking on me I got everything dripping with CLP to see if that would help, but that's not to say that CLP is a "good" oil. 

[NateDoge]

Well, I installed the Armalite extension kit and took the rifle out to test yesterday. The good news is that the rifle now functions and I did not experience a single malfunction. Unfortunately, the BCG now has enough travel to go back far enough for the gas key to knock the charging handle back each shot, so I have a reciprocating charging handle. That is obviously kind of obnoxious. I may have to alter the extension tube so that I can screw it in further. Not a big deal. Fortunately, the BCG is not striking the lower or otherwise causing damage, it just needs to stop a bit further forward.

Considering that the PSA extension was 1/8" longer than the Armalite extension and was not allowing the BCG to go back far enough to feed the next round or lock back, the PSA spring when compressed must be long enough to totally stop the buffer before it reaches the end of the buffer tube. So what I need in combination with the longer spring with fewer coils is an even shorter extension. 

At least the weapon is semi-automatic now! Just a little more tweaking til I get it right.

I would like to thank you guys for your help and suggestions. 

Edited April 16, 2019 by NateDoge

[shooterrex]

Just stack a few quarters behind the spring till the bolt carrier just comes back enough to engage the bolt catch.

[98Z5V]

1 hour ago, shooterrex said:

Just stack a few quarters behind the spring till the bolt carrier just comes back enough to engage the bolt catch.

Shouldn't have to do that on an Armalite setup - it's constructed the same everytime.  This is the second time I've seen one (recently) that might need to go in another turn, though.  Might have to notch the bottom a little, to clear the buffer retainer, when it's another full turn in.  Very light cuts with a Dremel Chainsaw Sharpening Bit does the trick, in short order.  Take a little off - test it.  Repeat if necessary.  Just a small notch is all it takes, nothing giant, or you ruin it.

Pics of what it looks like right now will help tremendously. 

[NateDoge]

I can't take pictures yet because I'm at work, but it could definitely go in another turn, and I will definitely have to create a notch for the buffer retainer in order to get it there. I just stopped turning it when the little lip on the bottom got to where it was holding in the retainer, which is not flush with the opening in the receiver. 

[98Z5V]

You could use a needle file - the notch only needs to be as wide as the pin, on the end of the buffer retainer, and the notch wont have to be deep.  Just a little at a time.

This is the second confirmed Armalite AR-10 carbine extension that has a lip on the bottom, and needs to go in another turn, but won't.  Looks like the new norm.  I think I'll order one up for testing.