I'm looking for advise before I start throwing parts at this gun. I have an dpms platform .308 that I just finished building. Here's the build:

24" BSF barrel
Fail zero NiB BCG
Aero precision gas block and tube
80% upper/lower
Cmc trigger
Radian charge handle/safety
Magpul sniper stock
Buffer/tube/spring from 80% kit

Buffer is 2.5" long and weighs 3.6Oz
Spring is 11.25" relaxed with 29 coils.

I tried to break in the barrel but had feed issues. It ejects brass just fine, right about 3 o clock. Bolt is held open every time when single round is fired. But when I fired with full mag, the bolt slams into the side of the brass and jams up. I continued to shoot just to break in the barrel, shoots one round at a time without issue.

I feel like I need a heavier buffer to slow things down. Any advice?

does it seem like the bolt just isn't quite low enough to strip off the next round and feed it?

I have had this problem and have gone through numerous BCGs, measuring them and also trying to raise the mag in the lower by way of the mag catch. I have tried numerous lowers with other uppers and they just seem a blond one off.

I gave up and have not had them back out in a while.

I will be watching this.

What length gas system?  If it’s rifle length you might need an adjustable gas block.  

Are the mags new?  Try loading only two or three rounds in the mag to see if it cycles then.

It is a rifle length gas system. Looking into it, an adjustable gas block might be a good idea even if it doesn't fix this problem. I think I'll try that first.

The mags aren't exactly new, but they have hardly had ammo in them. Been sitting in my closet for a year, empty.

I'm curious why you are using a rifle gas system with a short 2.5" DPMS style 308 carbine buffer.  If use a rifle extension and regular rifle buffer, the buffer weighs 5+ oz, which is considerably heavier.

I never really thought about that. I had the parts left from another build.

My adjustable gas block showed up yesterday, gonna try that out today. Hopefully that's the trick

Download your magazine by two rounds and see if it runs better.

Can you see where the rounds are jamming? Do the feed ramps and barrel extension have sharp edges? Is there a lot of slop in your magazine well fitment?

I use custom heavy buffers from Slash. I think AR-10's need them, not everybody is onboard with that. They are not cheap.

I never set rifles up with adjustable gas blocks. I want them to run reliably as is. If your rifle is locking open on an empty magazine and ejecting shells around 3:00 o'clock your gas is probably fine.

Ammo makes a huge difference in reliability. I refuse to buy steel case ammo for anything other than my $400 10/63 which I ride hard and put up wet. Every other firearm I own gets quality chow.

I use 55 foot pounds minimum when installing AR-10 barrel nuts. 70+ pounds is common. Big frame AR's need higher torque ratings.

Usually the bullet makes it up the ramps and into the barrel extension, the bolt then slams into the brass. Leaving the bullet tweaked in the brass.

The barrel has an m4 extension and I filed down the receiver to match. It's pretty smooth without any places to hang up.

The mag well seems ok, all my mags fit as well as any of my other AR platforms. I did have to put a slight bend in the mag catch because it didn't hold the mag in that well. Not sure if that is an indicator of something.

I shot it this morning with the adjustable block. No improvement. I gassed it way, way down and there was a slight improvement, but the bolt would no longer hold open on last round. Still it would only feed 1 out of every 5 or so.

Is it worth buying a rifle extension and buffer or I'm I just throwing parts?

I'm about to give up on this damn thing, but I have to much invested at this point.

I only recently found out both my AR308 gas tubes were too short.   Wilson Combat has the proper ones.

So after messing with this gun, I found that it cycles fine if I hold upward pressure on the magazine. The tiny bit of slop is enough that the bolt isn't stripping the next round.

Any solutions to this? Do I need to replace the lower?

If it’s an actual DPMS upper and lower, simply replacing the lower may not work.  DPMS is notorious for tolerance stacking, and there’s a significant chance that both the lower and upper are out of spec, or at least at the extreme outer edge of their specs.  DPMS also doesn’t sell matched pairs of receivers, in that they don’t test receiver sets for fitment and tolerance like some other companies do, like Aero, SLR, etc.

If it’s another company, or a mix of receivers, you’re also dealing with unknown variables for fitment.  

That being said, did you try to increase the buffer weight and spring weight?  Or install a gas block?  Have you attempted to slow down the bolt?

I installed an adjustable gas block and also did a rifle length buffer assembly. No changes.

So I took it to a gunsmith and he said the mag catch opening is milled to low. I called the manufacturer of the lower and they said it's been too long since I purchased the lower and they won't do anything for me.

I have a solution and I'd like some feedback from experts. I'm planning on building up the mag catch with weld, then milling it down to correct the .030" inch error in the lower. Then I would open up the slot in the lower for the larger mag catch.

Any thoughts?

The tubs flat wire buffer spring is always my go to spring when I change out gas blocks to adjustable gas blocks . And a stock carbine buffer works just fine.http://www.davidtubb.com/#tab-videos
https://www.youtube.com/watch?v=8fMVZbrnvu8

I finally got some time to mess with this thing again. I built up the mag catch with weld and filed it down to .024" above spec. Now it shoots without issue. Got maybe 200 through it and no problems.

Had same issue, it new and tight, oil it till is. Fucking slick,  tske the bolt apart soak in oil over noghfb clean re oil and problem solved and do not start messing with buffer and all that crap or you’ll be chasing shit forever, the big ARs especially need to be an oil slick