Am I correct to think the first thing to investigate is the gas system???

I'm using CPD and ASC mags of various capacities.Failure rate is about 10% with either brand and always the same......bolt is on top of the next cartridge in the magazine.I don't think the bolt is retracting far enough to pick up the next round.

I have a second 7.62 carbine and it functions EVERY time with the same ammo and mags.Ammo is Wolf Polyformance 123 gr HP.
Both guns are using standard weight buffer and standard carbine recoil springs.
Thanks for your help....

Tried swapping lowers yet?

Bird

What barrel length, gas system length, and gas port diameter on the problem gun?

How did you install the gas block, taking into account alignment front-to-back?

Both are 16" barrels,carbine length gas system ,standard weight buffer,standard carbine spring....
Port diameter unknown.....

Im thinkin its in your Gas block area as well..perhaps the
port size is smaller than you other barrel or misaligned some..
Start there.

Well-lubed?

You could start replacing parts from the working carbine with the non-working, but before doing that, I would make sure the gas block is aligned along the bore axis, not necessarily the rotational axis.

Your gas tube is installed correctly into the block?

Another thing could be a tight chamber that doesn't like to release the spent case.

A quick way to tell would be to drop in the BCG from the working gun and see what happens on the working lower.

If that fails, take apart the gas system and measure, then re-install.

Polish the chamber.

If it works, it's your carrier and carrier key or bolt.

Load 1 round in the magazine. Chamber. Fire

Does bolt lock back?

EVERYTIME I have a new upper this is the 1st thing I test for.

Scrub the crap out of the chamber. Poly steel ammo is dirty and needs to be cleaned more.

Lots of lube on the bolt carrier group?

I had to open the gas port on my 7.62x39mm rifle gassed 20" barrel to .125" to get 100% reliability. I did this in gradual steps, disassembling the rifle and using the next larger diameter drill until it would stay locked 100% of the time. .125" is about maximum because the interior of your gas tube isn't much different from that.

I don't use comblock ammo. IMI and PMC brass reloads with 24.5 grains of H4198 and Hornady .310" 123 grain SP's or Sierra's .311" 125 grain Pro Hunters seated at 2.190".

Sometimes I use 28.0 grains of H322 with these bullets.

The magazines available for this round almost always require some tweaking to get them running reliably. I use a grinder to remove metal from the front of the magazine well to ease the feeding process. Anything that slows the smooth operation down can effect reliability.

Since you're using the lightest carbine buffer the bolt should lock back on an empty magazine 100% of the time. Have you tried that test? Simply load one round in the chamber and fire the rifle with an empty magazine in place to insure it locks open on the empty mag.

I try to run the heaviest buffers that will lock the magazine to the rear every time when empty. It's easier on the firearm.

I would tend to agree with fellow ARF-ers', avoid using steel cased commie ammo, but if you do you will need to clean the chamber area more often and keep the chamber
clean and smooth as the steel cased ammo tend to be hard on the stainless steel barrels/chambers and they will over time develop ruff spots...and for this reason I like
to and recommend either 4150 or 4140 CMA steel barrels in this caliber as well as high pressure (with) smaller caliber bore diameters to help tame throat erosion and bore
wear...

As a side note, for example, AR-15 YHM barrels in x39mm caliber use .114"/.115" gas port diameters in the rifle length gas position because of the timing of the gas/port
pressure length, dwell time, and bore volume, etc.

Steel cased ammo tends to be under powdered for the AR-15 gas system especially the rifle length gas system and it tends to be pretty dirty and tends to gum up the
chamber area which in turn reduces reliability. Also, steel cased x39mm ammo tends to have shitty accuracy even out of a AR barrel...Then, I wish someone would
make a magazine in this caliber that is reliable right out the box for the standard AR platform w/o the need for tuning...Finally, the bolt and firing pin seems to need
or is prone to some form of Issue (although all these thing have Improved over the years as demand has risen). So yeah, rolling your own bass cased ammo is the
best way to go for optimal reliability and accuracy in your AR...

So bottom line is (seemingly) that while the x39mm cartridge was never designed for the AR-15 platform it can be made to function pretty good if the end user is
willing to take the time to make some tweaks and get everything nice and smooth.

Good luck to all those who love this caliber in their AR's.

Been running the latest version of the CPD mags in all 3 of my 7.62x39 AR's - so far 100% reliability with 5, 10 and 30 rd mags running more than 500 rds total. All 3 are carbine length gas systems with .100 gas ports - 2 with carbine stocks, 1 with an A2 stock - standard springs and buffers. One of the carbines did initially have cycling issues after it was built - traced it down to a rough chamber that a good polishing took care of. I do keep an eye on the chambers.

These guns are now just as reliable as my 5.56 AR's - great for hog hunting.

Bird

Gas block misaligned?

I had similar problem. Found my mag springs in 2 ASC mags were installed backwards, causing front of follower to go up and bolt to go over rear of cartridge. Re-installed, no problems since.

I would start with the gas block.

THANKS to each of you for your tips and advice!
I'm just swamped right now but hope to experiment further in the near future......
Regards