My 300blk SBR CMMG will not fire

[Triathloncoach]

I've always had trouble getting the magazine to catch. It easily catches when the upper is removed. Is the bolt in the way? My mechanical skills are poor. Now with a cartridge chambered I pull the trigger and all I hear is a click. I the bolt looks clean, I cleaned it again, without disassembling it. I tried different ammo.

 

[fast306stang]

cmshoot

 

[eyemikeb]

Don’t fill the magazine fully. In a similar sbr I have to have 25 or less in a 30 round mag. Works every time with less ammo. Good luck. Hope it’s something easy

 

[Mr. Booshie]

Is it a factory build? Or did you source all of the parts and assemble it.

 

[rOmiLaYu]

Have you ever fired the rifle before? If it has fired before and doesn't fire now, the issue could be different.

Off-hand, the "hey dummy" answer might be, are we sure there's a firing pin in the rifle? What about your trigger springs? Are they installed correctly? If not, you could be getting a light primer strike....which causes me to ask, are there any marks on the primer of the round that chambered?

All AR mags, when loaded, will have some level of difficulty beating seated with the bolt forward as the top round in the mag is pressing into the bolt carrier. With the bolt back or the upper off, insertion is easier.

 

[Tactical Beaver]

Does the magazine catch easily with the rifle assembled and the bolt locked to the rear? Is it a factory build or brand new rifle? Is the hammer spring installed correctly?

 

[Moondog1]

Don’t fill the magazine fully. In a similar sbr I have to have 25 or less in a 30 round mag. Works every time with less ammo. Good luck. Hope it’s something easy

Why? and why would an SBR be different than any other AR?

 

[Plinker66]

Blackout rounds are heavy on an AR mag spring. Test everything with just a few rounds in the mag to rule out the mag spring.

 

[eyemikeb]

Why? and why would an SBR be different than any other AR?

No idea. 2 pistols and 1 of my SBRs have similar issues. One pistol is cerakoted and I thought that might be why but not filled mags fixes it.

 

[cmshoot]

Not uncommon to have a mag catch issue when:
1. Bolt is in battery
2. You’re using an aluminum USGI mag and it has 30 rounds in it.

In order for the mag to catch, the carrier has to push the rounds down in the mag. There ain’t any room for the rounds to compress any further. That was one of the reasons why we only loaded them with 28 rounds in the Corps. The slightly longer mag body of the PMag solves this issue.

Failure to fire could be a lot of things.
-Make sure the entire BCG is assembled correctly (firing pin ain’t moving)
-Make sure the trigger group is assembled correctly (light strikes)
-Make sure barrel is installed correctly (barrel not seated, firing pin not reaching primer)

If you have an AR that works properly, swap complete upper and lower assemblies and try both new guns. If one doesn’t work, that narrows your issue to the upper or lower assembly. Then continue to swap parts to narrow the issue down.

I’m happy to help you in person. Give me a holler.