Built a suppressor - now breaking bolts.

[Ragedracer1977]

I built a suppressor for my grendel. Prior to the suppressor, I had about 15-1600 rounds through the gun. Mounted the suppressor and the bolt broke after about 100 rounds. Thought maybe it just wore out, so I ordered a couple new bolts from Underground Tactical. Broke another one after about 100 rounds. I currently have a bolt from Maxim in there, at about 100 rds and it hasn't broken yet, but I turned the gas way down (JP adjustable block) so that the bolt barely locks back on an empty mag.

Is this a known issue? My loads haven't changed. Only difference is the suppressor.

[LRRPF52]

Welcome aboard. Did you have an account here before with a similar username? It seems familiar. Many were lost after the last attack.

Violent extraction from too much gas, combined with excessive and imbalanced loading on the lugs adjacent to the extractor can cause this. I seem to recall another forum member who had the same issues with bolts from Les Baer.

The AA Hard Use bolt has a balanced lug arrangement.

What load are you shooting?

What barrel do you have, what length, what gas system length?

[lead chucker]

I shoot suppressed exclusively now, I must have just over a K through mine now and no problems. But I do use a adjustable gas block and keep it turned down to where the bolt will just lock back.

[LRRPF52]

There are some manufacturers who won't honor warranties if you use their bolts with suppressors, and will not honor replacements if their bolts are used in op-rod driven guns either for a reason, and that's talking 5.56 NATO.

Did yours break at the cam pin hole, or the lugs?

[Ragedracer1977]

LRRP: Yes, I've been a member here for over a year, but just got my account reactivated. I guess it was deleted.

16" BHW barrel, carbine length gas system. 123gr SST over 27.5 grains of Ramshot TAC.

Both bolts I've broken, have been at the lugs. The original bolt that was in there was a Les Baer .264 LBC bolt. The second bolt, as I mentioned, was a Underground Tactical. I have another UT bolt as well as PF supermatch as spares. Currently I have a bolt from Maxim in it. Both the Maxim and the PF have "rings" milled around the bolt behind the lugs. I have to imagine it's some kind of gas issue as it's the same load I've been shooting for almost 2 years on the LBC bolt.

[ricsmall]

definitely a gas issue. the problem should go away now that you adjusted the gas. Large suppressor manufacturers have the technology to reduce this back pressure and still remain relatively quiet. Ive been doing a bunch of reading on it and plan to build one later on a trust. All my F1 money keeps going to new lathe tooling, barrel blanks, etc. Keep us posted on the results. Care to show your design and some photos of process and finished product? If not I understand, just looking for ideas for the future.

Richard

[LRRPF52]

Back pressure and increased cyclic rate are trying to rip the case out of the chamber when the brass is still adhered to the walls.

For the LBC bolts, I think there were some metallurgy issues that are hopefully resolved. Les Baer will take care of it, but IIRC, when folks called with broken bolts, their first question was, "Are you shooting suppressed?"

Your load is 1gr lower than max published loads, so you pressure is not that high. The Carbine Length Gas combined with a suppressor is putting insane amounts of dwell time on the system, which is very hard on bolts. That's a known combo to cause reduced bolt life with 5.56 bolts as well.

One of the nemesis's of suppressor designs is the initial chamber formed by the first baffle, and how to keep that down. A lot of innovation in the market has transpired over the years, so I can see how a home-build would encounter this.

[Ragedracer1977]

definitely a gas issue. the problem should go away now that you adjusted the gas. Large suppressor manufacturers have the technology to reduce this back pressure and still remain relatively quiet. Ive been doing a bunch of reading on it and plan to build one later on a trust. All my F1 money keeps going to new lathe tooling, barrel blanks, etc. Keep us posted on the results. Care to show your design and some photos of process and finished product? If not I understand, just looking for ideas for the future.

Richard

I don't have any diagrams or anything. I used a tube and endcaps purchased from SD Tactical Arms. Then formed freeze plugs into cones using a hydraulic press. It's what I would consider hearing safe with the Grendel.

[Ragedracer1977]

Back pressure and increased cyclic rate are trying to rip the case out of the chamber when the brass is still adhered to the walls.

For the LBC bolts, I think there were some metallurgy issues that are hopefully resolved. Les Baer will take care of it, but IIRC, when folks called with broken bolts, their first question was, "Are you shooting suppressed?"

Your load is 1gr lower than max published loads, so you pressure is not that high. The Carbine Length Gas combined with a suppressor is putting insane amounts of dwell time on the system, which is very hard on bolts. That's a known combo to cause reduced bolt life with 5.56 bolts as well.

One of the nemesis's of suppressor designs is the initial chamber formed by the first baffle, and how to keep that down. A lot of innovation in the market has transpired over the years, so I can see how a home-build would encounter this.

Good to know that the suppressor is the likely culprit. Now that I have the gas adjusted lower, I'll see what happens. I'll see what transpires over time with it.

[Variable]

Yeah, I'm guessing backpressure too. Running a tight bore will give you great sound, but it drives the pressure up greatly during cycling.