Broke another bolt.... your thoughts??

[Cajuntransplant]

My guess is the bolt is normal. I'd first check uneven mating of extension to barrel, or uneven mating of barreled extension to upper receiver. All this exacerbated by increased pressure of suppressor on top of large-ish port in mid-length gas system on an 18". Sort of a perfect storm. And if your loads really are a little warm. . . .

Yup. I’m going to run factory ammo through it to check speeds when I can. Then pull the scope and will be taking apart the upper to check things over in the next week or so as I get time. Will see what happens. I’ve seen other 18” Grendels on here chrono factory ammo at close to, equal, and even slightly over my handloads. I know that doesn’t mean they are the same with powders and everything, but still want to see where my setup pushes factory ammo at.

[Cajuntransplant]

Well, this snack sure has turned into a three-course meal.

Your loads are hot and hammering those bolts. If you can square the lugs and share the load you might get another 500+rds before the next one cracks. Or...you can take your foot off the accelerator and enjoy shooting that gun until the barrel wears out.

Option A costs nothing.
Option B costs time, effort, probably money and does not address the cause.

All the more technical and involved solutions here are based on the assumption your loads are normal, and they are not.

Easy there Klem. People are sharing their thoughts. That’s it. Handloads could be hot, maybe I’ve got a fast barrel. I know I did my load dev up to and over what quickload and others chimed in that worked for them. I came down roughly .3 to .5 (in my notes somewhere) grains and found my sweet spot. Yup, it’s fast and it’s stupid accurate out to 880 yards. Others have logged factory ammo same speeds or more as my handloads in 18” barrels through these forums. I’m gonna run factory thru it to compare, then take things apart and see what’s what. My gun. My handloads. My risk. My experiment. I asked a question to start this off and got a lot of answers. Some I never even thought of before cause it’s never been an issue with 5.56 stuff. I’m taking everyone’s answers and suggestions on here into consideration. I’ll sort it out and report back when / if I find the cause.

[Mopardoctor]

I have found that not all Grendel bolts are equal. I measured the inside of the bolt from the tail to the back side of the bolt head on my new AA bolt and Satern bolt and they matched. Then I measured a BCA 6.5 Grendel bolt I got on sale and the inside measurement is .080" longer so the bolt head on the BCA bolt is .080 thinner. A .223 bolt measured the same as the BCA bolt. I suspect the BCA bolt was made from a .223 bolt blank. You get what you pay.

Mopar Doctor

[Klem]

Easy there Klem...

I find people here tend to overlook the obvious as an excuse to make more of a technical meal and discussion out of something that is usually simple. Sometimes online status and kudos is involved. Others (including you) might be playing down the obvious to save pride and out of respect. You say it might be a 'fast barrel'. Well of course it is...when you overload your cases with too much powder. I'll bow out of your thread now and let you pull your gun apart and measure things.

Meanwhile, your friend with the 20" barrel with velocities slower than your 18" has yet to blow a bolt, yes?

'Easy there' Cajun, at least with the loads.

[grayfox]

Klem's just looking out for your safety, guy. His point is well taken but you will address it as you shoot and chrony some factory loads. I had an 18" that was fast but it was finicky. Broke one bolt in it too. Sold it off b/c it was so picky, but it ran ~50 fps faster than expected, more like most 20" barrels ~2515 for 123sst's whereas 2475 was more expected... come to think of it the load felt "hot" when I pulled the trigger, so --- we're just sayin'. Could be it was a hot load, I don't know but it was more trouble than I wanted...
I think Lrrpf52 had some observations on 18" barrels and Suppressors being hard on bolts also due to the extra pressure and blowback... so that could be an aggravating factor for you. Prime causes IMO seem to be load-hotness (which you can verify) and non-square upper/barrel matchup...

Stay safe fella. Slow is better than maimed.

[centerfire]

Klem nailed it first try. Ejector swipes ever fourth or fifth round plainly means you have excessive pressure. You don't have a fast barrel, you have a set up that conservatively shows pressure signs. You're getting the velocity of a 20" barrel because you are probably loaded around 60k psi. CL is a great tool for estimation but should only be used to start a load, not predict absolute pressure. Your broken bolts have clearly told what is going on, you've just ignored the actual problem. You're handloading a cartridge that does not tolerate excessive pressure, prolonged pressure signs will (have) yield catastrophic failures. Grendel should be worked up to the correct velocity, not pressure like most other cartridges. If you want to push the cartridge you need a stronger platform such as a bolt action.

[LRRPF52]

Yeah I was mistaken... It’s 18” and mid length gas. Don’t know gas port diameter. Whatever they punched it at is covered by adjustable gas block right now. Never bothered to check gas port diameter.

Satern normally cuts a .0935" gas port on MLGS 18" for some reason.

That is very large for that port location, and causes early unlocking with too fast of a cyclic rate unless you really find a way to tame it down with choked gas and heavy buffering, unsuppressed. That's a tall order without the can as it is. With the can, you will break bolts.

A MLGS 18" Grendel shouldn't have a port larger than .076", unless you plan to only shoot 95gr and lighter bullets with fast powders.

Upper is aero precision with a satern machine 1-8 twist 5R barrel, 18", Mid-Length Gas
Has an adjustable gas block from superlative arms, silencerco omega suppressor

18"...check
MLGS....check
.094" gas port....check
Suppressed....check
Broken bolts....check

The same thing will happen if you port a 5.56 with a .094" port MLGS 18" suppressed. This is one of the reasons why they went RLGS with a reflex can with the SPR. Push that port as far forward as possible, and allow plenty of initial chamber pressure relief before going through the baffle stack.

Now we're introducing more charge weight into the equation, with much heavier bullets, but then we have some relief with larger bore volume, but not enough to stop high cyclic rate with such a large port.

Will happen with

[Cajuntransplant]

Update for everyone that was awesome in talking things through with me and helping me understand everything. I understand everyone's thoughts and respect everyone's opinion as well. Didn't mean to come off in another way. Anyways...

Went to range this morning and ran 15 rounds of factory Hornady Black 123ELDM ammo and 15 rounds of my handloads. Used magnetospeed chrono and would take one shot, wait 30 sec, take another shot til I had shot five rounds and get the info and reset chrono and let supressor get cool to the touch then continued. Nice cool morning in low 70s with a south wind.

FACTORY AMMO - average speed from the three, 5 shot strings - 2488fps, all brass showed high pressure ejector marks

HANDLOADS - average speed from the three, 5 shot strings - 2490fps, 3 of the 15 rounds showed ejector marks

Don't know how many loads were on this brass for my handloads. I run the brass until primer pockets are shot or I get a split when resizing.

Oddity is the factory ammo ejected forward at about 2-oclock and handloads were consistent at about 4/5-oclock. Standard deviation from the factory was average of 17 between the 3, 5 shot groups. Handloads had an average of 5. Looking at a lot of the comments on here from respected members of the forum I'm pulling the barrel this week (hopefully) to send it off to see what the deal is. I checked my notes and my velocity of over 2500fps was in may / june of this year and probably just damn hot out. Hot temps, hot supressor, hot ammo. The shop that sold me the barrel and bolt wants it back since this is the second bolt they've given me and has broken. I'm going to see if they will swap it out for a 20" with rifle length gas system instead to prevent problems like this in the future.

[Gusmeister]

Update for everyone that was awesome in talking things through with me and helping me understand everything. I understand everyone's thoughts and respect everyone's opinion as well. Didn't mean to come off in another way. Anyways...

Went to range this morning and ran 15 rounds of factory Hornady Black 123ELDM ammo and 15 rounds of my handloads. Used magnetospeed chrono and would take one shot, wait 30 sec, take another shot til I had shot five rounds and get the info and reset chrono and let supressor get cool to the touch then continued. Nice cool morning in low 70s with a south wind.

FACTORY AMMO - average speed from the three, 5 shot strings - 2488fps, all brass showed high pressure ejector marks

HANDLOADS - average speed from the three, 5 shot strings - 2490fps, 3 of the 15 rounds showed ejector marks

Don't know how many loads were on this brass for my handloads. I run the brass until primer pockets are shot or I get a split when resizing.

Oddity is the factory ammo ejected forward at about 2-oclock and handloads were consistent at about 4/5-oclock. Standard deviation from the factory was average of 17 between the 3, 5 shot groups. Handloads had an average of 5. Looking at a lot of the comments on here from respected members of the forum I'm pulling the barrel this week (hopefully) to send it off to see what the deal is. I checked my notes and my velocity of over 2500fps was in may / june of this year and probably just damn hot out. Hot temps, hot supressor, hot ammo. The shop that sold me the barrel and bolt wants it back since this is the second bolt they've given me and has broken. I'm going to see if they will swap it out for a 20" with rifle length gas system instead to prevent problems like this in the future.

Good move I think. Sometimes you just have to chip away at a problem. Thanks for sharing and being the seed for an excellent discussion!

[Cajuntransplant]

Satern normally cuts a .0935" gas port on MLGS 18" for some reason.

That is very large for that port location, and causes early unlocking with too fast of a cyclic rate unless you really find a way to tame it down with choked gas and heavy buffering, unsuppressed. That's a tall order without the can as it is. With the can, you will break bolts.

A MLGS 18" Grendel shouldn't have a port larger than .076", unless you plan to only shoot 95gr and lighter bullets with fast powders.



18"...check
MLGS....check
.094" gas port....check
Suppressed....check
Broken bolts....check

The same thing will happen if you port a 5.56 with a .094" port MLGS 18" suppressed. This is one of the reasons why they went RLGS with a reflex can with the SPR. Push that port as far forward as possible, and allow plenty of initial chamber pressure relief before going through the baffle stack.

Now we're introducing more charge weight into the equation, with much heavier bullets, but then we have some relief with larger bore volume, but not enough to stop high cyclic rate with such a large port.

Will happen with

Thanks for the info and background on the mid versus rifle length gas info. I’m gonna see if I can do an exchange for the 18 with mid to the 20 with rifle length.

[centerfire]

Are you getting hard extractor marks on your brass?

[Cajuntransplant]

Are you getting hard extractor marks on your brass?

No. Just some ejector swipe marks once in a while on handloads. Factory ammo had ejector swipe every piece.

[centerfire]

How long is the throat? Early unlocking /over gassing is normally accompanied by hard extraction.

[Cajuntransplant]

How long is the throat? Early unlocking /over gassing is normally accompanied by hard extraction.

Not a clue. Sending it off to be replaced / traded out from the guys I bought it from. Never had extraction issues.

[Cajuntransplant]

Question for anyone still following this thread here....

If I can only do a direct exchange for my barrel and bolt and end up replacing it with the exact same thing.. then whats my next step to prevent the problem if it's based off the cyclic action being too fast and harsh for running suppressed? Since the factory ammo and my handloads are running same speeds and factory stuff shows a lot of high pressure signs, do I need to change buffer weights / springs to try to control the harshness of the cycling to eliminate the issue of breaking bolts? From what KLEM, LRRPF52, GRAYFOX and others have chimed in on here I seem to have an issue where (even the factory ammo) is too fast for my rifle setup and causing issues. I haven't pulled the barrel yet since I've been so busy with work and all, but if I can only do a direct exchange for the same setup (satern type II, bolt head, Mid Gas, etc) then won't I still have the same issues down the line unless I address how hard the gun is cycling? Taking handloads out of the equation... Factory ammo was averaging within just a few fps of my handloads but had ejector marks on every piece of brass, high SDs, a large ES, and erratic ejection patterns. Even if I get exchanged out and run same setup and only run factory ammo won't I have the same issues? Do I need to get a heavier buffer and then retune my gas system? Ideally I am going to try to exchange it for a 20" with rifle length gas and go from there, but if that isn't an option, what do I need to look into changing?

[A5BLASTER]

For what's it worth I have mild ejector swip on all brass no matter if it's a handload or factory and that's with two 18 inch JP barrels a 20 inch GB barrel and my 16 inch AA barrel.

Doesn't matter if I run normal weight parts or lowmass parts both with adjustable gas leave mild ejector marks.

I don't even worrie about it anymore.

If you can exchange the barrel for a 20 inch rifle gas from whatever you have now I would.

You can also play with different buffer weights and spring strengths with the adjustable gas and see what it does.

[Cajuntransplant]

For what's it worth I have mild ejector swip on all brass no matter if it's a handload or factory and that's with two 18 inch JP barrels a 20 inch GB barrel and my 16 inch AA barrel.

Doesn't matter if I run normal weight parts or lowmass parts both with adjustable gas leave mild ejector marks.

I don't even worrie about it anymore.

If you can exchange the barrel for a 20 inch rifle gas from whatever you have now I would.

You can also play with different buffer weights and spring strengths with the adjustable gas and see what it does.

Appreciate the info. I’m hoping (sort of) that theres something off with my barrel and after I get it replaced I won’t have the issue anymore but I’ll just have to wait and see what I can get it exchanged with. Hoping I can swap for a 20” with rifle length and then be good to go.

[A5BLASTER]

Appreciate the info. I’m hoping (sort of) that theres something off with my barrel and after I get it replaced I won’t have the issue anymore but I’ll just have to wait and see what I can get it exchanged with. Hoping I can swap for a 20” with rifle length and then be good to go.

Give us a update when you do more sir.

[Cajuntransplant]

Give us a update when you do more sir.

Will do. Hoping to get barrel pulled off this week yet and maybe by next I’ll have time to inspect it well and see if I notice anything before sending it out. Once barrel and everything is exchanged out and I get it back together I’ll do the same tests with factory and some handloads and see how things look.

[centerfire]

Silencers don't increase chamber pressure significantly. You're not breaking lugs because of dwell time or port pressure (adjustable gas block). I'd wager you have/had a short throat. Measure the throat on the new barrel and load accordingly.