My 300 Blackout has never given me any issues before, any mag any ammo. (never sub-sonic) I haven't had it to the range in a while and I just added a hyperfire EDT to it so it was time.

Loaded a Hexmag up and it went bang okay. But I noticed the charging handle was back (Crosshill sidecharge) The pictures show what was happening. Three different Hexmags and two pmags. 3 different types of ammo. It looks like the bolt starts to push the round forward but as soon as it hits the lower part of the ramp it stops. This happens mostly on the first two rounds or so but has happened in the middle of a mag. I changed the buffer spring which was about the coil and a half shorter than a new one. Helped a bit but the bolt still stuck some. As a last resort I replaced the new trigger with the old one. This helped the most but the bolt still stuck sometimes.

Is there anything else I can look for? I'm thinking the feed ramps could be polished or something. I can't see how replacing the trigger could start all this.

What length gas system? Pistol or Carbine?

Carbine

Hmmmm.... I don't have a .300blk but I was looking into one. Everything I was looking at was stating that a pistol length gas system was the way to go.

 

The cartridge was designed to be fired out of short barrels, like 7"-10", and therefore the powder fully burns faster. With the powder burning up so fast, the extra distance from the chamber to the gas block can cause the guns to run undergassed - which is what your malfunction sounds like, an undergassed gun. Now your gun ran before, but not now, so I am like 50% on that being your issue.

 

Another possible culprit is a lack of M4 feed ramps in your upper receiver. I see your barrel extension has them, does the receiver continue them?

Yes it does. 

 Wouldn't undergas cause short stroking? The gun ejects every round and locks back on the empty mag fine. The trouble is feeding the next round. Sometimes. I can simulate the problem by just racking the slide and letting it go. 

Undergassed guns can cause nosediving of the next round in the mag if the BCG drags and doesn't have enough "umph" to clear the top round and get behind it to strip it from the mag.

 

Failure to lock back on an empty mag is the standard test for undergassed. If it is passing that, you're probably good there and it's something else.....

That link was for the model 24 so it didn't help much. Plus I put the old mil-spec trigger back in and the rifle still does it. Even just manually racked. (which always makes me nervous)

are you getting this nose diving issue with other brand mags as well (pmag / GI)?  I had a similar problem last year and it was a bad mag with the feed lips spreading apart (hexmag; not sure which gen).  it caused all sorts of feeding issues.

You mentioned you have not taken it out in a while.  Maybe the BCG is dragging a bit and some oil is necessary.

are you getting this nose diving issue with other brand mags as well (pmag / GI)?  I had a similar problem last year and it was a bad mag with the feed lips spreading apart (hexmag; not sure which gen).  it caused all sorts of feeding issues.

Three different Hexmags and two pmags. 15-30 Hexmag, a 10 and a 15 pmag. So I don't think its the mags.

 

You mentioned you have not taken it out in a while.  Maybe the BCG is dragging a bit and some oil is necessary.

The RO and I thought of that so he lubed it up for me. 

I'm building another AR so I'll swap lowers to see if thats the problem. If not, Bob from Cheyenne Firearms says he can meet me at the range to see whats what. Fresh eyes and someone whos built a few ARs

I usually put adjustable gas blocks on my 300 rifles. I had a carbine laength gas system on a 14.5" + comp barrel and had to use a JP adjustable buffer to get it to cycle correctly. I now only have short barreled 300 rifles because that's what they are intended for.