When do Grendel AR bolts break?

[muvef]

You didn't have a grendel, you had some kind of mutt barrel.

Agreed, JSE f@%k$d me. It was advertised as a SAAMI spec chamber that required a .136" bolt face. I was pissed, especially after the crap I had to deal with trying to get the bolt replaced.

[LRRPF52]

Milspec steel for M4/M16 bolts is 8620 with specific heat treating requirements. A number of manufacturers use different steel alloys and different heat treating so it definitely is not a one size fits all scenario.
Additionally, 5.56 is rated for up to 62,000 cup while 6.5 Grendel is 52,000 cup, so there is a significant difference in pressures at open time. The further down the barrel the gas port is located and the size of the gas port also affect unlock pressures.

Mil-Std steel for M4 bolts is Carpenter 158, not 8620. They then compromise the bolts with non-destructive testing with the HPT procedure, causing microscopic cracks in them. The Canadians at Colt Canada, engineers at Knights Armament, and Bill Alexander all feel that is a terrible testing approach to use on bolts.

[Fess]

As I recall, Bill Alexander said that the bolts he used had features to help ensure strength and longevity. I think that one was a larger diameter chamfer inside the bolt at the base of the lugs. There may well have been additional features. As far as I know, there has been no SAAMI-like specification to ensure bolt uniformity, so some bolts may well have a areas of higher stress concentration and more of them.

Does anyone have more detailed information (or a better memory)?

[Texas]

Mil-Std steel for M4 bolts is Carpenter 158, not 8620. They then compromise the bolts with non-destructive testing with the HPT procedure, causing microscopic cracks in them. The Canadians at Colt Canada, engineers at Knights Armament, and Bill Alexander all feel that is a terrible testing approach to use on bolts.

You are correct, the bolts were carpenter 158 and the carriers were 8620 - so much for my memory.

[FLshooter]

I’ve never tried shooting my AR with the gas block shut off and I never would.I got a homemade brass catcher that works perfectly.
That being said I’ve been shooting ARs for a long time and I own a couple dozen.I’ve had several ejectors break because I liked using the cheap steel cased Russian stuff.AndI’ve changed out a few gas rings.I had a firing Pin or two crap out. But never have I had a bolt break.
I keep an extra bolt for each caliber and a bag of 10 firing pins just in case but like I said never had a problem w/a bolt and very seldom a pin.But,I am a prepper.
My Colt 6920 patrol rifle has maybe 5000 rounds through it .I’ve had it for 15 yrs?I clean it often. It’s one of the things I like to do ,I like to clean my guns .I guess it’s an excuse to handle them.I like to handle my guns.