Beautiful day went out to run some rds through a 350 legend upper I built recently and out of 19 rds fired at least 7 or 8 failures to feed. On most the bolt dosen't catch the rim and closes on an empty chamber or doesn't catches the cartridge past the rim and jams when it enters at a hard angle.

I'm guessing this is all related to the magazine and not a gas issue?  But I did run 6 rounds through the mag by hand before I shot them up with zero problems.

What say the hive?

Wilson combat barrel cmmg magazine Winchester 145g  FMJ

I only have the one mag at this point but will be looking for more before my next range trip.

My LR-308 pistol build did this same thing too me at first as well.

I also initially believed it was a magazine issue.  Working on that end I found that putting a wood block in the bottom of a 25-round P-mag UNDER THE SPRING such that the magazine spring was highly compressed and the magazine held like only 8 rounds would work. (This after trying a lot of other stuff first).

This made me realize the gun was cycling so rapidly and violently that the rounds either didn't have time to rise up enough for the bolt to catch them on it's way back forward and/or the cycling shock was such that the rounds were being jostled in the mag such that only very heavy mag spring pressure was sufficient to keep them up tight enough to the top.

So I changed out the standard AR-15 pistol buffer tube with the two weight shorty LR-308 buffer for an A5 pistol buffer tube with an H3 standard three weight buffer.

This solved 90+% of the issues with that gun (same symptoms as your having) when using the 25-round P-mags stock full capacity without the wooden block in the bottom to increase mag spring pressure (and unfortunately reduce capacity as well).

Then I changed over to using 20-round C-products thin wall stainless steel metal magazines in that gun which have both thinner feed lips which let the round present a little higher and also stiffer magazine springs to start with then the P-mags with still good capacity.  That got me to like 98+% reliable function.

Finally, I swapped out the basic gas block for an adjustable one and that combined with everything else puts me at basically 100% function and reliability.

Hope it isn't as big of a headache to get your gun that although a different chambering has the same symptoms mine had.

But I would suggest heavier buffer to start with.  If you have a lower with a A1/A2 rifle buffer system in it I would suggest throwing that upper on to the rifle buffer system lower and see if that solves the issue.  If you only have standard carbine buffer system lowers available then try dropping in an H3 buffer.

Long story short if your gun is doing the same thing mine was then you need to slow down your bolt cycling speed to keep it from doing that.  Adjustable gas block that you can turn down a little might be something to explore as well.

Good luck !!!

More than one thread on this topic.  Mine does this with the deer season ammo, but feeds fine with the white box ammo. Same magazine, just different ammo.

I have a bear creek upper using cmmg mags. 100% so far with ball and hunting loads. Have not put a whole lot of rounds through it yet though.

I’ve read that adding a small half moon notch to the front of the mag solves most of these issues.  Perhaps you could switch bolts for a few rounds to see if there is any improvement.
As an aside I have built 5 350s so far and yet to see one that was over gassed. My thought is that the pressure curve for straight case caliber like the 350, and to some extent 450 BM is much sharper and shorter than a bottle neck case. In one build I used an Odin works AGB and a lightweight BCG and only had to close it off a few clicks before it wouldn’t lock back. One needed the gas port opened up to function properly. My point is I believe most of these are right on the edge of having enough gas to work properly.
Try a few hunting loads which run hotter to see if they work better.  If they do you have just diagnosed your gas issue.

The case of the bullet is catching on the mag. Put a little half moon and it should fix it.

I've heard that works for a lot of people and I'll try it but most of the time the bolt is closing on an empty chamber because it doesn't catch the rim (least that's what it looks like) so I doubt that helps

Your bolt is outrunning your mag. Start with a heavier buffer. Then go to an adjustable gas block if the buffer does not help.

I'll try that thanks all for the suggestions