When you shoot enough to break

[338lapua]

Your 9mm AR bolt. This is the 5th one of this particular one that I have had. They guy I bought it off of replaced the other ones but this time I am going to a Faxon bolt where the faux gas key is not bolted on it is part of the bolt. I got about 50K on this bolt before the screws snapped after the last time I replaced the screws and re staked the screws. Still trying to find out who makes this bolt so I can make them replace it again.

The broken one is made by R&D precision, The first ones were not even staked. This one the screws kept coming loose so I replaced them and used rockset on them.

 

[Tenring]

Dammm!

 

[338lapua]

Dammm!

The upper is ok. Funny how the Colt bolt carriers go forever but these new ones break.

 

[BigNate]

A few thoughts...
1) Are there any witness marks on the back of the gas key - from it impacting the buffer tube / extension? If so - that would certainly make life harder on the screws
2) When you replaced the screws did you get them from a source that you trust to use the type of material appropriate for purpose? Just saying that cheap screws made from cheap(er) material would be more prone to failure... If I'm putting that many rounds through that gun - and have had to replace the gas key - I'm going to figure out what Bravo Company, or Faxon, or some other premium parts manufactures use - and use those screws.
3) (said with a smile)... If you put 50,000 9mm rounds through that BCG with those screws in it... they've lived a good life... 50,000 rounds is somewhere between $8,500 and $20,000 in ammo (depending on whether you roll your own on one end - or get out of your helicopter in the parking lot at Bass Proshops to buy boxed ball ammo in bulk at retail prices - also assuming that you aren't plinking with Federal HSTs - or we can make that top number much bigger)... So... go buy your Faxon BCG and be happy...

Lastly - if you can't figure out who made the BCG to get it replaced - and you are going to toss it - I think it could be kind of interesting to try to TIG it into place and see if that can be made to work. If you are in the valley and want to try that - let me know. Odds of it working? Slim... Odds of it being a learning experience? Pretty good...

EDIT: - got curious - did some digging - found this thread with a bunch of good info in it - including some indications that TIG welding the key to the carrier is an option, and recommendations for a couple of different screw suppliers. https://forums.brianenos.com/topic/261015-gas-key-bolts-sheared-off/

 

[338lapua]

It did not strike the buffer tube, I had enough spring power to stop it from getting there. New screws were sourced from a gun parts company. I rarely shoot factory ammo, I don't even use factory ammo for defense ammo. I am halfway through a 25,000 round run of 9mm. The problem is the previous carriers that never made it to 3K rounds, first one broke in less than 2K rounds. I believe the problem to be the weight of the faux gas key slamming back and forth and it probably damaged by it slamming to a stop when chambering a round. This is the colt bolt in 9MM bolt and it uses the same gas key as the AR-15 the 5.56 uses. I own several Colt 9mm rifles and one is in excess of 150K rounds since I owned it. The other 6450's are probably approaching that number. The point is I have never had a bolt failure on any of them. Only reason I dont shoot the Colts all the time is the magazines are hard to load where the Glock and the H&K mag lowers are easy to load.

I think when I send it back I am just going to get them to remove the broken screws then I will put it together with a regular gas key and stake it myself and see how that does. My bet is that it does not break again.

 

[delta6]

........................This is the colt bolt in 9MM bolt and it uses the same gas key as the AR-15 the 5.56 uses.


______________576064 copy.jpg

What.. you sure??????????????

 

[BigNate]

That thread that I linked indicates that the mass of the faux gas key is a generally accepted factor in this sort of failure - along with too little spring / too little buffer mass resulting in the key hitting the tube. One of the poster's (the surly guy) either own or is instrumental in the creation of specialized BCG solutions for 9mm AR Platform products - including this fella which minimizes the gas key to the maximum possible so as to simply / only keep the BCG from rotating...

https://taccom3g.com/product/extreme-short-stroke-9mm-bolt-essb/

 

[delta6]

Nate,
The bolted, welded gas key makes some sense. The only reason there is a gas key on a blow-back 9MM Colt type carbine/SMG, is so you can charge it. If it become that big an issue, mill the upper, drill a hole in the carrier, tap/pin a cocking handle and the problem is solved. Not sure how the screws shear? There is really no interaction of the carrier with...anything? Maybe the back and forth, but that is not very dynamic, since it is buffered in the rear and cushioned in the front.

 

[338lapua]

........................This is the colt bolt in 9MM bolt and it uses the same gas key as the AR-15 the 5.56 uses.


______________576064 copy.jpg

What.. you sure??????????????

I posted the wrong one

 

[hairygreek]

I wonder if the screws were hardened like grade 5 or grade 3 bolts.

 

[338lapua]

I wonder if the screws were hardened like grade 5 or grade 3 bolts.

Grade 8. Maybe they were too hard??

 

[338lapua]

Found this last night and this makes sense as it does not have the mass of the other keys, going to grab one and try it too.

https://taccom3g.com/product/standard-9mm-bolt/