Firing Issue

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04-18-21, 15:15
daveblack

.308 Build/Fitment/Firing Issue

First post here. Hope the knowledgeable members can help me with my build. This is my second build, but I am not 100% sure what is going wrong.

Built a .308 with these parts:
-Aero upper
-DPMS Gen 1 lower (non-Aero)
-Faxon Firearms Low-Profile Gas block (fixed)
-Heavy Rifle Buffer (9oz)
-Armalite Buffer Spring
-Fixed rifle stock
-Luth-AR 308 Bolt Catch

There is a gap when I connect the upper and lower. You can see where the Aero upper hole does not align with the rear takedown slot. I can get the rear takedown pin only by hammering it in. Picture is with the front takedown pin only.
https://i.postimg.cc/yWGnpj8f/IMG-20210418-153111.jpg

I have tried a different upper and I have the same fitment issue.

When I fire it, the round ejects, but the bolt only goes back halfway and the next round doesn't feed right. Can pull bolt all the way back with the charging handle to reload manually.
https://i.postimg.cc/m1sNYXwk/IMG-20210328-155425.jpg

Any ideas?
04-18-21, 16:44
dpete

Your fitment issue is due to there not being any milspec or other dimensional spec. for 308 ARs. Every manufacturer can make any piece any dimension they want. Put your Aero upper on an Aero 308 lower and that issue will disappear. The best method is always to buy upper/lower from same company to avoid this. If the bolt only goes back 1/2 way when fired it sounds like it is hanging up somewhere on its way into the receiver extension. Not hard enough to completely seize but enough to prevent full function. Can you feel any drag at all when you pull it back by hand?
04-18-21, 17:13
Black_Sheep

If it has the same fitment issue with a different upper, it's probably an out of spec lower.

If you read through my "disappointing outcome" post, you'll see that I had similar feeding issues and read about how it was resolved. It sounds like your rifle is undergassed, but there are some basic checks you should do before you start changing things. Start by checking bolt gas key, gas block alignment and gas tube protrusion into the upper.

Knowing your barrel length and gas block diameter would be helpful.
04-18-21, 18:51
grizzman

My Aero uppers fit on my old DPMS lower. It does sound like a problem with the lower, not the idea to use them together.
04-18-21, 22:30
pinzgauer

Aero M5s are DPMS "LR" pattern and should match other LR pattern.

The one I had worked great.

I can't remember specifics on DPMS generations though.
04-18-21, 22:39
daveblack

Quote:

Originally Posted by dpete

Can you feel any drag at all when you pull it back by hand?

Thanks for the response and good question. Yes, it drags more at this point and right before I have it fully pulled back.

Quote:

Originally Posted by Black_Sheep

If you read through my "disappointing outcome" post, you'll see that I had similar feeding issues and read about how it was resolved. It sounds like your rifle is undergassed, but there are some basic checks you should do before you start changing things. Start by checking bolt gas key, gas block alignment and gas tube protrusion into the upper.

Knowing your barrel length and gas block diameter would be helpful.

20" barrel, 0.750" gas block. You drilled out the gas port and added a heavier buffer tube?

Quote:

Originally Posted by dpete

Your fitment issue is due to there not being any milspec or other dimensional spec. for 308 ARs.?

Quote:

Originally Posted by Black_Sheep

If it has the same fitment issue with a different upper, it's probably an out of spec lower.

Quote:

Originally Posted by grizzman

My Aero uppers fit on my old DPMS lower. It does sound like a problem with the lower, not the idea to use them together.

The fitment makes sense. Any chance I could have a gunsmith fix it? These were 80% lowers, otherwise I would have bought the same brand. It looks like the slope doesn't start early enough at the back. Not sure if this is the cause of firing issue.

I have this same problem with an identical upper/lower combo. The bolt gets caught when I pull the handle all the way back. It slams forward when I hammer out the rear takedown pin.
04-19-21, 15:52
Black_Sheep

Quote:

Originally Posted by daveblack

Thanks for the response and good question. Yes, it drags more at this point and right before I have it fully pulled back.

20" barrel, 0.750" gas block. You drilled out the gas port and added a heavier buffer tube?

The fitment makes sense. Any chance I could have a gunsmith fix it? These were 80% lowers, otherwise I would have bought the same brand. It looks like the slope doesn't start early enough at the back. Not sure if this is the cause of firing issue.

I’d try to resolve the fitment issue first to eliminate bolt / buffer misalignment as the issue.