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!