Aero M5 Bolt Stuck Locked Back

[Dellisaurus]

Anyone have thoughts on this?

Sometimes my M5 gets its bolt stuck fully to the rear HARD. So hard I have to hit the bolt catch with a nylon hammer. Only seems to happen at the end of a mag. I don't have any photos currently but it doesn't look different than regular lock back. There was initially an issue in the first few rounds through the gun where the BCG was very hard to pull back with the handle. That seems to have gone away.

All parts are well oiled. There are wear marks in all the normal spots on the BCG. Some wear on the cam pin head.

I am a Sig certified M400/AR-15 armorer and armorer for a law enforcement agency, so I've seen lots of issues but this has me stumped. This is not an agency gun.

Rifle
Aero M5 upper and lower
Faxon 18in .308 barrel
Faxon BCG with headspace matched bolt
Odin works adjustable buffer
I don't remember the brand of the bolt catch, I think it's a CMMG.
RA drop in trigger
Aero ambi changing handle
Happens with Amend2 and Hexmags but the mag always drops free while the bolt is stuck.

Any thoughts?
Thanks

[4V50 Gary]

swap buffer?

[tangolima]

Check the set screw on the buffer "hatch". It may have backed out and raised burr on the inside of the buffer tube.

-TL

Sent from my SM-N960U using Tapatalk

[FrankenMauser]

How old is it?
There are rumors that Aero completely lost control of QC when they moved to Tacoma.

[stagpanther]

By saying it locks back hard at the end of your magazine I'm inclined to think what you probably have going on is an issue with the bolt catch and magazine clearances IMO. When it happens does the bolt catch on the magazine step itself--or is it instead wedging the magazine follower somehow? 308's are not standardized like AR15's--so there could be other issues contributing to the problem (I'm a little suspicious of the adjustable buffer thing). First thing I would do is a carrier/buffer check--simple press the carrier assembly back into the buffer tube--the carrier should bottom out with the back squared off area right about at the point of contact with the rear ears of the lower. If there are wear spots on the lower's ears that look like the carrier might be impacting them you likely have a mismatch in the tube/buffer system.

Aero's xx 308 receivers are great components--but it's not uncommon for them to not quite match up quite right with other manufacturer's. The bolt catch configuration they use might be off just enough that when mated with a different manufacturer's it might not operate reliably. It's a pretty complex system between the springs, pins and cutout-outs in the receiver--and they also can wear in time. For xx308 ARs I've found it really pays to invest in top-quality as hard as possible heavy-duty bolt catch assemblies. That's what I think is the highest probability of the root cause of your problem. Springs or pin gone in the bolt catch assembly could conceivably cause an issue like you describe (I think). Hitting it with a hammer would certainly run the risk of wasting that system if it wasn't already broken.

Other things that might be worth checking is simply how true the carrier runs inside the upper--and for the heck of it pop your forward assist pin out and make sure that might not be hanging on the carrier somehow.

Good luck and may the force be with you.

PS--just noticed that you are using a CMMG catch--I've bought many CMMG lower parts kits for lots of AR builds--but for an xx308 I would spend time searching the internet for a top-quality one--especially one that's made of hardened steel and one which the manufacturer specifically says optimized for use in xx308 platform.

PSS--sometimes the release slot in the magazine might position the magazine too low or too high in the well-that also might be worth investigating.

[Dellisaurus]

Originally Posted by tangolima

Check the set screw on the buffer "hatch". It may have backed out and raised burr on the inside of the buffer tube.

-TL

Sent from my SM-N960U using Tapatalk

Is that the screw that is below the safety in the lower? I wasn't sure what that screw does.

[Dellisaurus]

Originally Posted by FrankenMauser

How old is it?
There are rumors that Aero completely lost control of QC when they moved to Tacoma.

Very new, built it this winter.

[tangolima]

Originally Posted by Dellisaurus

Is that the screw that is below the safety in the lower? I wasn't sure what that screw does.

No it is the set screw on the buffer.

-TL

Sent from my SM-N960U using Tapatalk

[Brian Pfleuger]

I would try loading and firing a single round with and without a mag and see what happens.

[SLR15]

There is no mention of what type of length of buffer tube, buffer, or buffer spring used (action spring), if you could advise it may also help.

The screw you are referring to, is it the one under the grip that is right next to the grip screw hole? If so, this is for tightening in to push on the lug of the upper receiver, which causes the rear of the upper receiver to go up and take up the slight wiggle between the upper and lower receiver. If you have this screw, then back it all the way out or remove it and see if your issue goes away, as the problem you may be experiencing is that the back of the upper receiver is pushed up, and now the bolt carrier is running at a downward slope forward, rubbing on the bolt catch, I have seen this often on many problem guns that are brought to the Armorer courses we teach.

Next take a look at the top of your bolt catch to see if there is any damage or rubbing that would indicate that you either a bad bolt catch that is too high, or a mix match of parts where the tolerances aren't working correctly together, in the machining world this is referred to sometimes as stacking.

CY6
Greg Sullivan "Sully"
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SLR15 Rifles
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