why would bolt break?

[montana]

The reason the Grendel bolt is weak is because they took a 5.56 bolt which was designed for the 222 Remington, reamed it out leaving less material to fit the 7.62X39 base used in the Grendel. If your upper receiver is out of spec, your bolt wasn't heat treated correctly or you shoot high pressure loads you will get bolt failure. LRRPF52 is correct when he said to adjust your gas system with the correct buffer, recoil spring and the correct gas port diameter for the barrel length. I use my adj gas block to "fine tune" my different loads. I had one gas port come from the factory over sized and my adj gas block corrected it. I think we may be over thinking the adj gas block. Use the correct buffer, recoil spring, gas tube, gas port size for barrel length and your good to go.

[bigbear_98]

I called les Baer and told them my bolt broke. First thing she asked is if I was running a suppressor. "Yes" she said don't do that. The suppressor caused it to break. The gun can't be set up correctly to run a suppresor and they don't recommend it. I will always break bolts as long as I run one. I asked for a better explanation, but she just would say the gun can't be set up right for a suppressor.

That being said, I really like their customer service. All of the conversation was very cordial and polite. The experience I have had with them and krieger is Sooo much better than some others I have dealt with.

[LRRPF52]

The original Colt 7.62x39 bolts, then AA Beowulf bolts (which became the Grendel bolt) were and are made as dedicated bolts from scratch. Reaming a 5.56 bolt will not work for a number of reasons if you use the deeper face recess, since there needs to be more material on the tail of the bolt to set the firing pin up.

I think the bolts in question broke because they are 8620, and the heat treat made them too brittle. There aren't a lot of other explanations for why a bolt would shear lugs after 2-3 boxes of factory ammo. With such a small geometry bolt in the AR15, with the larger case head, you really need good alloys with highly-competent heat-treaters to make a long-lasing bolt.

These 8620 bolts have been breaking in 5.56 AR15's for years and years, especially when the owner shows up to a high-volume carbine course with one. Sheared lugs are the most common breakage, followed by cracked cam pin holes.

For those of us with AA bolts, not even hard-use ones, dating back for years and years, they keep trucking along. You can break them if you run a diet of loads that are over the edge, but staying under 51,800psi peak pressure will help you avoid this.

I have broken a bolt in a 5.56 gun I used for 3-gun. It had over 10,000 rounds through it, ranging from Wolf, M855, and Federal, to Winchester. Broken bolts were an occasional occurrence in certain CQB courses at Bragg, because of sling tension on non free-floated guns once they heated up.

If the alloys and heat treat QC are given more attention to detail than mass-produced bolts for the VISMOD 5.56 AR15 market (Look, I have an AR15!), a substantial Grendel bolt is common and not that big of a deal. Once you remove one of the pillars from that formula, then breakage is likely, just like on an 8620 5.56 bolt.

[montana]

[QUOTE=LRRPF52;79286]The original Colt 7.62x39 bolts, then AA Beowulf bolts (which became the Grendel bolt) were and are made as dedicated bolts from scratch. Reaming a 5.56 bolt will not work for a number of reasons if you use the deeper face recess, since there needs to be more material on the tail of the bolt to set the firing pin up.

The Grendel bolt is a dedicated bolt made to fit in the parameters of the Rem 222 barrel extension, and upper receiver. As you can tell I didn't do well in composition, LOL . Thanks for clearing the confusion up.

[Tedward]

Firing pin is the same and protrudes more on the Grendel Bolt

Quote from LRRPF52: "Reaming a 5.56 bolt will not work for a number of reasons if you use the deeper face recess, since there needs to be more material on the tail of the bolt to set the firing pin up."

LRRPF52, if I'm not mistaking the bolt face on the Grendel Bolt being .010 deeper lets the firing pin protrude further than the .223/.556. I measured them and confirmed this with a few other people. You can actually look at the two bolt faces with the firing pin pushed forward and the Grendel bolt/firing pin has more pin protruding.

Reaming out a .223/.556 bolt removes the shot peen surface weakening the bolt so it isn't a preferred process but some have done this and with non over loaded ammo, they have been working fine.

I think the most important thing is the material and correct heat treating in conjunction with correct pressures on the loads being used. Even the strongest bolts manufactured correctly will fail with loads that are over pressured.

Just the firing pin part threw me off but maybe I'm reading it wrong, if so I apologize in advance...

[bigbear_98]

Quote from LRRPF52: "Reaming a 5.56 bolt will not work for a number of reasons if you use the deeper face recess, since there needs to be more material on the tail of the bolt to set the firing pin up."

LRRPF52, if I'm not mistaking the bolt face on the Grendel Bolt being .010 deeper lets the firing pin protrude further than the .223/.556. I measured them and confirmed this with a few other people. You can actually look at the two bolt faces with the firing pin pushed forward and the Grendel bolt/firing pin has more pin protruding.

Reaming out a .223/.556 bolt removes the shot peen surface weakening the bolt so it isn't a preferred process but some have done this and with non over loaded ammo, they have been working fine.

I think the most important thing is the material and correct heat treating in conjunction with correct pressures on the loads being used. Even the strongest bolts manufactured correctly will fail with loads that are over pressured.

Just the firing pin part threw me off but maybe I'm reading it wrong, if so I apologize in advance...

So is les Baer just machining 556 bolts to 65?

[LRRPF52]

No, that is the reason why a Grendel bolt with a deeper face recess needs a longer tail, so you can use standard firing pins.

One way to find out if the person who made your bolt did the same engineering or not is to simply measure the overall length of the bolt after you have measured the bolt face depth of .136" (-.003" allowable, no +).

If your bolt is 2.810" long, then you likely have an AA bolt meant to be used with standard AR15 firing pins. If your bolt OAL is the same as a 5.56 bolt: 2.800", then they didn't increase the tail length to accommodate firing pin protrusion.

I just checked this down at my bench with several bolts:

* 5.56 bolt: 2.800" long, .124" face recess, .280" lugs
* AA Grendel Bolt: 2.810" long, .135" face recess, .278" lugs
* "Grendel" Bolt sent to me for free by a forum member years ago: 2.802" long, .1373" face recess (out of spec headspace), .280" lugs
"Grendel bolt" that came with a higher-end barrel maker's combo: 2.800" OAL, .1373" face recess, .278" lugs, firing pin has a pit right on the tip.

So if you're getting excess firing pin protrusion by 10 thousandths, there is the possibility you will pierce earlier. This is the first I have noticed the pit on the firing pin on the other BCG that I thought had a bolt made to spec.

Interesting what you learn when you break out the calipers, and closely inspect your parts. There are other obvious features on 5.56 bolts and AA Grendel bolts that aren't on the others, like the circular cuts behind the lugs at the rear lug roots, which is in the TDP for M4 IIRC. I think the M16A1 used a slash relief cut in the rear root of the lug, while they later went to a circular relief cut.

I just went through about 12 different AR15's to compare, and there are a lot of ones that don't have it, while my Colt MPC bolts do have it.

So is les Baer just machining 556 bolts to 65?

I doubt someone would do that with a reputable company. None of the major names in this business make their critical parts, because they don't have the capacity to do the heat-treating. Bolt manufacturing is out-sourced to the few shops that know how to do it and have kilns. They do it to the specifications provided to them by the customer, in this case, whatever company name you are familiar with, through the ABC's of AR15 makers.

[bigbear_98]

One way to find out if the person who made your bolt did the same engineering or not is to simply measure the overall length of the bolt after you have measured the bolt face depth of .136" (-.003" allowable, no +).

If your bolt is 2.810" long, then you likely have an AA bolt meant to be used with standard AR15 firing pins. If your bolt OAL is the same as a 5.56 bolt: 2.800", then they didn't increase the tail length to accommodate firing pin protrusion.

* AA Grendel Bolt: 2.810" long, .135" face recess, .278" lugs

I just measured and both Les Baer bolts are 2.810" overall, .135" recess, .278" lugs. None of the broken bolt looks damaged, except the two sheared lugs.

I ordered a stripped bolt from Les Baer, installed the other bolt I had from them, and ordered two of the PF bolts.

I still don't feel like I have an answer for any of this. I did learn a lot though. Suppressors cause a longer period of peak pressure on the bolt face. Les Baer seems to think this is why the bolts break. I dunno. I think I am going to install my suppressor, adjust the gas block until I don't see as much soot on my cases, and shoot it.

[LRRPF52]

I have shot a lot of suppressed in the most extreme conditions you can subject a rifle to:

Extreme cold temps, high humidity, high volume, gun starts at ambient -27 C temperature, run the gun through drills until hot, gun rapidly cools down to -27 C, repeat, x 2-3 days. Not only that, but I watch a lot of other guys do it. Granted that is with 5.56 and 7.62x39 guns, but I had a Rock River carbine that I punished like that for years. I shot it suppressed with an Ase Utra CQB QC suppressor in high volume courses in extreme cold like that. My pistol grip broke, and the detents corroded, but everything else was fine. It's barrel and bolt were changed out after over 10,000 rounds, but were still intact, functional, etc.

Do your bolts have a circular little cut behind the lugs, moving backwards along the boreline?

[BjornF16]

how about pics of the bolt?

[bigbear_98]

I've shot a lot of 223/556 suppressed and not. Lots of 3 gun as well, but I don't think it's the same thing as the 6.5.

how about pics of the bolt?

[Tedward]

LRRPF52, thanks for the info on the measurements.

Looks like tomorrow I will be pulling bolts and measuring. My LBC bolt is the one that had the firing pin protrusion greater than the .556 so I'll be going thru the ones I have left on hand. I had 5 different 6.5 Grendel manufactures of bolts earlier this year but never measured the OAL. Things people tell you are just unbelievable. I posted the firing pin protrusion issue earlier this year and this never came up. The topic was about light primer strikes on my 556 but not my Grendel, more research....

[SHORT-N-SASSY]

First things, first:

1, I'll take this opportunity to thank all of you for your invaluable input to this matter that concerns so many.
2, "We shoehorned a bigger cartridge into the gun. . . . Now impose a Grendel geometry, and you are right at the tipping point.": an honest statement that applies, here.

As I review this Thread, I'm impressed by the friendly professionalism that underscores the Replies. And, considering the many variables involved --- some, of which, are beyond our control ---- each, with the ability to exceed "the tipping point," I'm equally impressed with, what would appear to be, a choice/desire/need to continue this tight-rope act, aware that a "fall" is but into the near creek, and not into the abyss.

In view of all the above, a fair question: Isn't it time 6.5 Grendel Enthusiasts accept the fact that the standard AR-15 Bolt isn't making it; i.e., without significant modification/upgrading? And, finally, will we still be hashing this old problem, a year --- two years, from now?

[bigbear_98]

Is it that we don't want to leave the ar 15 or do we have too much invested to try to change platforms? I wouldn't mind a different receiver, if I could move my barrel, magazines, and trigger to it. Is it possible to change the extension to make the bolt bigger?

I still think this is an awesome cartridge.

[LRRPF52]

AA made a decision to use the AR15 receiver set for these cartridges. With the Beowulf, it requires a larger ejection port like the other big-bore uppers do. No matter which way you look at it, you have to decide what the market will be able to support.

* With the Grendel shoe-horned into the AR15, you have massive market access because of parts commonality.
* If you decide to build a new receiver set, then why stay constrained to the AR15 mag well COL limitations?

At that point, you are the sole-provider, and you have to convince machine shops to run way more special parts now, unless you think one business has the capacity to make all their bolts, bolt carriers, barrel extensions, magazines, special handguards, etc.

So in order to make it accessible with the most popular rifle design, the fewer unique parts, the better. With the Grendel, that is the bolt, which already has a reputable source for it from the Beowulf production chain, that is properly heat-treated, inspected, and passes industry standards based on engineering work done on it. In short, it already works well.

You can't blame the originator who is doing their part to crank out quality parts that hold up to the pressures, when others who have set out to make their versions work using a different set of standards, results in failed components.

Right now, as long as the metallurgy is done correctly, you have a perfectly functional system that enjoys commonality of parts to the AR15. As soon as we depart from that, we then need new barrel extensions, new barrel nuts, and new upper receivers at a minimum. Why stop there?

The demand for Grendel performance from the AR15 is only going to grow, primarily because of hunting, followed by target-shooting. The best thing for manufacturers to do is to get on the ball on their materials sourcing, heat-treatment, and other critical design specifications.

The bolt in the picture above does not even have the relief cuts behind the lugs that are common to 5.56 bolts, just like many other "AR15" manufacturers have been doing for decades now. Somewhere, someone who specs out the dimensions for their parts is either unaware of the TDP, or chooses to ignore it for whatever reasons, and I'm talking about 5.56 guns here.

With the Grendel and even 7.62x39, there are some engineering challenges that need to be addressed, and AA has done this from the start. Precision Firearms has also been cranking out reliable, durable Grendels, as has JP. I have talked with shops that aren't even mentioned on this forum, like Specialized Dynamics, who caters to a lot of coyote hunters. Number one caliber request: 6.5 Grendel

Precision Firearms has several calibers he builds for: 5.56, 6.5 Grendel, and 6.8 SPC.
Number one caliber request: 6.5 Grendel
Think about how many other shops out there are getting requests for Grendels that we haven't heard of.

I think this deal with the bolt group buy has a lot of potential to establish a regular source of quality bolts, that could blossom into a larger market. Numerous shops and dealers have already tried to buy the entire lot before they are even finished, so that should tell you something about demand.