300 blk build cycling issues FTF

[Quacker]

Put 20 rounds through my new 300 blk build this weekend. Had several FTF... trying to diagnose the problem but could use some help. I’ll try to list all the specifics but I’m sure I’m going to leave some out so be patient with me.

Aero precision receiver set m4 feed ramp
Colt BCG

Ballistic advantage 10.3” SS barrel with factory pinned GB and pistol length gas system. M4 feed ramp

Using a mission first 30 round mag

Without checking it right now I am not sure what weight buffer I have in it.

Shooting 150 grain rem SuperSonics FMJ-HP.

Started with 20 rounds in the mag. Would strip and chamber the first round fine. After firing the first round the next would chamber but not go into battery. After performing a Mortar clearing due to stuck cartridge I found the cartridge was severely gouged by either the bolt lugs or chamber lugs. Pic below. This happened about 6 more times. The last 4-5 rounds cycled great.

Dunno if it is short stroking due to heavy buffer or not being broken in or gas issues. Any thoughts are appreciated. Anyone seen brass like this before?

I don’t really think it’s a mag problem but I may be wrong. I put 20 rounds through a pmag the first time I shot it and had a few FTF but didn’t see brass like this.

I like to think I’m pretty competent and meticulous with my gunsmithing so let’s assume that going forward...

 

[Quacker]

Weird how it gouges like this but still chambers. Makes me think the gouges are coming from the chamber lugs and not the bolt.

When I extracted the case it looked like the gouges were on the extractor/ejection port side of the chamber.

 

[Buckeye76]

Looks almost like the mag wasn't seated all the way and the round was forced up.

 

[jeep78]

Try a GI mag before you change anything. Preferably a mag with a known good spring.

Seat the mag with an open bolt.

 

[Quacker]

Try a GI mag before you change anything. Preferably a mag with a known good spring.

Seat the mag with an open bolt.

I’ll give that a shot. Got some Okay GI mags tucked away.

 

[Dirty J]

Agree. Change mags. I don't know anything about that mag brand... but change it!

That 150gr round weighs 3x as much as a normal 5.56 round. A tall order for even good mags to get the rounds up in time. Add in a magazine of unknown reputation... ASKING for trouble.

 

[Plinker66]

You prolly need a dedicated blackout mag.

I had feeding issues until I went with metal D&H 20 rounders.

They are clearly marked also, which makes them safer.

20 rounders are lighter also; that blackout is heavy stuff.

 

[Johnny B. Goode]

I had the same problem with the Troy mags. It sat too low in the mag well. I'd say with 99.99% certainty it's a mag issue.

 

[Plinker66]

https://palmettostatearmory.com/d-h...-black-w-red-follower-300blk-al-10-bt-rd.html

The top of these is shaped different than the 556 mags, imo.

 

[sweathogger273]

Wouldn't hurt to swap to a extended/enhanced firing pin