explain this please pressures and bolts?

[Huntchic]

I'm having a hard time understanding 6.5 Grendel max pressure and potential bolt breakage vs other cartridge's .
If a 5.56 and few other AR 15 calibers have a higher pressure than the 6.5 Grendel how is it i keep seeing folks talk about Grendel being a concern of breaking bolts and parts? would seem since 5.56, 300 blk have higher max pressures and they share the same carrier just a different bolt why would a Grendel be more likely to break parts and bolts.

[grayfox]

Force=pressure*cross-sectional area. The Grendel's fatter case means the diameter of its base, and the diameter of the bolt face, are larger than the base area of a 556/BO/Hamr, etc. But they share the same lugs that have to bear that force. So to avoid too much force on the lugs, and thus avoid breaking them, you have to limit the max pressure of a Grendel to a lower value than for the 556-style cartridge.

[Huntchic]

I appreciate that grayfox. I had considered that the diameter of the case was possibly the reason.

[LRRPF52]

I’ve been shooting 5.56 in high volume since 1988, starting with a Colt Sporter II AR-15A2. Shot a lot of high volume in the Army, then even more once I got out and started competing in 3-Gun, and running CQM courses. I’ve put a lot of 5.56 carbines through tortuous range sessions in a lot of environments, the harshest being in the Arctic. I have only broken 5.56 bolts so far, cracked at the cam pin hole. I don’t think I’ve sheared a 5.56 lug, but have seen it.

I’ve been shooting 6.5 Grendel since 2009, mainly in a precision rifle and DM course shoot volume. That includes 16” MLGS, 22” RLGS, 20” RLGS, 18” MLGS, 17.6” MLGS, and 12” CLGS suppressed. I’ve been shooting the 12” CLGS suppressed the most for the past 6 years, with 17.6” MLGS and 18” MLGS LaRue as the alternate/alongside Grendels.

I’ve stacked boxes upon boxes of Grendel brass over the past 15 years, much of it used to reload and have yet to break anything but scopes. I’m at the range or running a 2-3 day course every month. On top of my own Grendels, I’ve seen scores of them come through courses where we shoot all-day for 2-3 days.

I have only seen bolt failures online, but have had friends say they broke extractors or bolts over the past 15 years in a few circumstances, just like people have broken 5.56 bolts.

If you have a 5.56 bolt subjected to the Military TDP where it is High Pressure Tested, it is more prone to break than how KAC and some other companies do their processes. HPT is one thing in the TDP I would get rid of. Canadians have a great process as well for their AR-15s.

I’m still trying to break a Grendel bolt. I though the 12” CLGS suppressed would be the best way to do it, but so far, no dice. It’s using a Monster bolt sourced from SixFive Outfitters/Rexus, before the newer Rexus bolt even. I’m also using a Bootleg carrier with adjustable gas to control the cyclic rate and keep it locked-up longer. Extra power action spring, standard carbine buffer.

[Huntchic]

I appreciate your input and experience LRRPF52.
My first year in Marines i spent in the Armory and saw about everything that could happen to an AR / M16 and other type platforms. Everything from them used as prybars to whatever. I've built a great many Civilian AR platforms and firearms between Muzzleloaders from scratch flintlocks to various bolt rifles etc. like you things do break sometimes and quality components defiantly matters. I've just been curious about the 6.5 Grendel because I've seen so much talk (and yes mostly on internet) about broken parts or cations on breaking things loadings more than any other caliber. Just left me scratching my head with Why would that be questions.

[LRRPF52]

Don’t tell anybody this, but you know what I did when I broke my first 5.56 AR-15 bolt? (Engage creepy Biden whisper voice...) I replaced it with.....another 5.56 bolt! It was crazy.

Now I still haven’t broken a Grendel bolt after all these years, but someday I may. And if I do, I’m going to....dump all my Grendels because they’re trash!!!!

Or maybe I’ll just drop in a new bolt like I did with 5.56. Sounds like a really tough decision though. (sarcasm)

Precious few people will ever shoot the round counts I have through their 5.56 AR-15s, let alone Grendels.

Great thing about the AR-15 is how easy it is to user-service and replace parts.

Stoner was a genius in that regard, which is why it has done so well in the military.

It’s an armorer’s dream compared to the M14.

[JASmith]

LRRPF52's experience suggests that using ammunition made to specifications is indeed bolt friendly.

If I recall correctly the discussions of Grandel boat breakage were more frequent in the early days when folks were trying to squeeze extra velocity out of the Grendel in attempts to make it look more like a 6.5 Creedmoor. Of course that meant pressures well past the SAAMI maximums.

Naturally, the folks doing it for the most part reluctant to admit that they shot extremely hot loads and preferred to blame the bolt.

(Of course, the breakage did necessarily happen with the "hot" load. In these cases, shooting hot loads significantly accelerated metal fatigue that led to breakage down the road. Hence, the perpetrators didn't make the connection. Even more interesting, is they might also have transferred the Grendel to another party who innocently experienced the breakage.)

[Huntchic]

I agree with you both. Sometimes people get a new cartridge and try to make it "bigger" than it was intended. saw this alot back when i got my first 10mm auto. people were trying to make it into a 44 magnum. I do load some calibers up around max charges and wont say ive not gone 1-3 tenths over in testing but you cant make a 243 a 300 win. pick the tool for its intended job.

[mtnlvr]

Like 9mm +P+?

[Huntchic]

Yep. Agreed

[rickt300]

I suspect there are far fewer broken Grendel bolts than discussions about broken Grendel bolts. I did break an extractor after running around a thousand rounds of steel cased Wolf through the rifle and possibly 400 reloads, of course some of them were a little hot as load workups,120 Hornady factory loads. Simple fix, got rid of the rubber Oring and so far no more problems.

[Huntchic]

Thanks for that input Rickt300

[LRRPF52]

There were gunshow-grade companies who tried to jump on the Grendel bandwagon back in the mid-late 2000s using their already-proven trash 7.62x39 bolts that broke lugs and extractors all the time, because they were .125” face depth and made with poor metallurgy.

Model 1 Sales was one of them. They made garbage 5.56 and 7.62x39 at really low price points to follow the volume of sales business model, low quality.

They called their imitation 6.5 Sporter. Those things broke as a rule regardless of chambering.

Only competent companies who care about their reputation should even attempt to get into AR-15 bolts. The 5.56 bolt is a little low on the mass as it is, so metallurgy and processes has to be tightly-regulated in production and inspections.

There are some new approaches that doubled the life of 5.56 bolts, as published by ARDEC.

My go-to bolt right now is the Rexus Arms bolts.

[LR1955]

I suspect there are far fewer broken Grendel bolts than discussions about broken Grendel bolts. I did break an extractor after running around a thousand rounds of steel cased Wolf through the rifle and possibly 400 reloads, of course some of them were a little hot as load workups,120 Hornady factory loads. Simple fix, got rid of the rubber Oring and so far no more problems.

Rick:

No so sure of that. I sheared lugs off of four or five bolts before the various enhancements to their design and metallurgy took hold. Personally I think the original Grendel bolt was a sheared lug waiting to happen. Since then there have been a lot of changes to them and now I don't think I have heard of a sheared lug for at least five years.

BTW

[Huntchic]

Well I’ve got a spare and crossed fingers.
I actually carry cleaning gear including a breakdown rod always as well as few extra parts in the field especially overnight trips.