First broken bolt for me!

[cst]

Hey guys Yesterday I was shooting my grendel and had my first ever bolt lug fracture since I built it 4 years ago.

19" Lilja barrel with midlength gas
Maxim\PF BCG nickel boron
SLR adjustable gas block
Surefire 762 Soccom

The build is about 4 years old with over 2000 rounds thru it and strictly reloads of:

123r. scenar
LVR 31gr.
MV 2550 at 80deg.

Its a slightly hotter round and its been shooting so well that I never even thought bolt breaks would even happen with me.
The Lug that broke was the one right below\adjacent to the extractor...Lucky for me I had a spare bolt and swapped it out and kept going.....

I had not idea it broke.....the only reason why was because the piece got jammed underneath the trigger and it wouldnt go off..lol

Considerations:

-LVR has a moderate MV change in temp changes so pressures could have spiked yesterday since it was 65deg in the morning and hit 95deg that afternoon.

-I know the surefire can adds a lot more pressure and I have only been using it the past 2.5 years

If i turn the gas down more....would that decrease chamber pressures so i can still use the hotter loads or should I just cut back the powder about .3gr....?


OR say F-k it! and keep on goin!?

Any thoughts?

[VASCAR2]

Changing the gas block setting will not affect the initial pressure of the fired round. Adjusting the amount of gas will dictate how vigorous the actions cycles. Adding a suppressor can affect the dwell time and back pressure on your 6.5 Grendel. I adjust my gas block so it locks back by the bolt catch on an empty mag. My spent brass land at 3-4 O Clock but I don’t use a suppressor.

Just curious if you lapped the face of your forged upper receiver or did you use a billet upper. It is suspected that if the barrel and bolt are not in alignment additional wear can be imparted on the bolt and barrel extension.

[cst]

It was a Seekins billet upper and Geissele Mk14 handguard. I usually lap but I cant remeber if I did for this build.

[VASCAR2]

Usually billet uppers are square so sounds like your components are top tier. I kind of look at bolts like tires on a sports car (wear item). You drive your sports hard you won’t get the tread life. Some manufactured items just last longer than another part off the same production line. I had an extractor fail after a couple thousand rounds and now just keep spares. My rifle was over gassed and I messed up a few hand loads where I had to mortar my rifle. I figure this might have shortened the life of the extractor but I’ll really never know for sure.

[LRRPF52]

The most important factors here are:

* 18" MLGS
* Suppressed

Cartridge comes after that. Run a 5.56 NATO 18" MLGS suppressed and see what happens.

SOCOM has experienced early parts failure for many years now due to suppressed use and gas systems that weren't meant to be suppressed.

The problem is that the bolt is under thrust when the increased cyclic rate forces the carrier to the rear, and now you have torsion on the lugs as well, with additional back-pressure.

First things to go are:

Extractors
Bolts

This is why SOCOM solicited the industry to make the Suppressed Upper Receiver Group for the M4A1 SOPMOD Block upgrade program. Instead of replacing parts all the time, they're dealing with it by spec'ing out a complete, integrally suppressed upper that is meant to run suppressed all the time.

Flow-through suppressor designs have been doing better with mitigation of cyclic rate and back pressure, allowing the cans to vent forward, rather than acting like accumulators.

If you look at the SPR, they ended up going with RLGS on an 18" .223 Wylde for this reason, so the plug dwell time would be minimal.

If I were shooting 6.5 Grendel suppressed with an 18" barrel, I would want RLGS or ILGS and an adjustable carrier for conventional can, or flow-though suppressor.

I'll be using my TBAC Ultra5 -30 5" can on my 12" CLGS Grendel, which will act more like a 14.5" CLGS Grendel-totally fine with the correct port and gas pressure into the action.

When you add what acts like barrel length to an already excessive plug dwell arrangement of the 18" MLGS, it's going to be very hard on the bolt with rotational torsion.