6 arc bolt troubles

[The boogalorian]

I have been working up a load for my ar15 in 6 arc for several months now. I have done ladder testing for Berger 105 hybrids, 105 bt's, 108 bt's, 109 lrht's, 90 bt's, and Sierra 107's with varget, re15, vv150, and blc2.
Rifle: Seekins Precision upper and lower, Craddock Precision 22" Bartlein barrel with +2 rifle gas system, brownells low mass bolt carrier and 6.5 grendal bolt, JP enterprises buffer with (2) tungsten weights, triggertech diamond trigger. Shooting suppressed.
I fired some (400) rounds of the factory black ammo and (200) of the match ammo. The black 105's averaged 2660 fps, and the 109 match ammo averaged 2550 out of my barrel. Factory ammo did have flat primmers and some of the cases showed thinning of the case wall to the point that i did not attempt to reload them. All chronograph data was taken with a labradar. Factory ammo accuracy was 3/4-1 moa.
The 90gn Bergers and Sierra 107's showed the most promising results during the ladder testing. I could get the 90's going about 2720 fps before seeing swipe marks on the brass. The 107's could be pushed to about 2550. I opted to go with the 107's averaging 2530 fps using varget and 205m's.
To date there was about (900) rounds down the tube. Went to the range this morning to zero the rifle and take it out to distance to straighten out my strelok. The range session was cut short before I could complete the zeroing. Broken bolt lug.



Going to replace the bolt with a JP Enterprises 6.5 grendal bolt and try again. Not sure why it broke. This barrel seams pretty slow compared to what most others are pushing their pills at.

 

[DaveTooley]

Pretty simple. As designed that bolt size can effectively handle a 5.56 round. Moving up to a Grendel/ARC size cases puts a load on the lugs it wasn't designed to handle. It works most of the time but not always. The bolt and extension need a new improved design.

 

[rebeltex]

That new JP bolt will fix your problem.

 

[243Mendoza]

wow, that bolt lug failed early. It took a thousand rounds with some hot loads to break a lug on my bolt.

 

[gunsandgunsmithing]

I believe I was told that I didn't know what I was talking about and my experience was questioned by another poster recently, on this exact subject. But who's counting? He was looking for a fight and I put him on my very short ignore list.

Check this link and you'll probably find the reason to be bolt thrust and fatigue. Either way, it's a great link with good info on bolt thrust and how it's calculated. The JP bolt is an improvement but the AA bolt is a pretty good bolt, too.

A Look at Bolt Lug Strength - Lilja

All action designers are concerned primarily with producing a safe and functional product. All other considerations such as weight, finish, eye appeal and other cosmetics are secondary. From a safety standpoint, the strength of the bolt lugs is of prime concern. Bolt lug shear strength depends...

riflebarrels.com

 

[DaveTooley]

wow, that bolt lug failed early. It took a thousand rounds with some hot loads to break a lug on my bolt

wow, that bolt lug failed early. It took a thousand rounds with some hot loads to break a lug on my bolt.

His broke at 900 rounds and yours broke at 1K so there is a problem.

 

[David Milisock]

I have been working up a load for my ar15 in 6 arc for several months now. I have done ladder testing for Berger 105 hybrids, 105 bt's, 108 bt's, 109 lrht's, 90 bt's, and Sierra 107's with varget, re15, vv150, and blc2.
Rifle: Seekins Precision upper and lower, Craddock Precision 22" Bartlein barrel with +2 rifle gas system, brownells low mass bolt carrier and 6.5 grendal bolt, JP enterprises buffer with (2) tungsten weights, triggertech diamond trigger. Shooting suppressed.
I fired some (400) rounds of the factory black ammo and (200) of the match ammo. The black 105's averaged 2660 fps, and the 109 match ammo averaged 2550 out of my barrel. Factory ammo did have flat primmers and some of the cases showed thinning of the case wall to the point that i did not attempt to reload them. All chronograph data was taken with a labradar. Factory ammo accuracy was 3/4-1 moa.
The 90gn Bergers and Sierra 107's showed the most promising results during the ladder testing. I could get the 90's going about 2720 fps before seeing swipe marks on the brass. The 107's could be pushed to about 2550. I opted to go with the 107's averaging 2530 fps using varget and 205m's.
To date there was about (900) rounds down the tube. Went to the range this morning to zero the rifle and take it out to distance to straighten out my strelok. The range session was cut short before I could complete the zeroing. Broken bolt lug.

View attachment 1346483View attachment 1346484

Going to replace the bolt with a JP Enterprises 6.5 grendal bolt and try again. Not sure why it broke. This barrel seams pretty slow compared to what most others are pushing their pills at.

Thanks for posting, this. I need to study your data as I'm going to be working some loads with Sierra 107's. I'm double checking as if my memory serves you 107's were pretty hot in a gas gun for Varget, Hornady lists 2450 in an 18" with 105's.

What brass are you using? Was the VV powder N150? The rest of your powders look pretty fast for the heavyweights according to Hornady data. Even the 90 grains seemed top or over the top, according to Hornady data. They list 2600 in an 18" with the 90's with Varget, however all their data is with their bullets. The bearing surfaces are probably different at the least.

Looking at your post Hornady factory which uses Leverevolution for their 105 and 108 projectiles, you pushed your 107's as fast as the Hornady load but with a faster powder. Is that correct?

 

[troupe]

I am not a engineer, and I do understand the cost involved in some of this. Why not make the bolt head a little larger and the barrel extension a little thinner. Seems like it could open up to even more cartridges. The barrel extension/bolt head I guess can't be made much larger on the AR-15 platform. I also know rounded corners seem to work better than the sharp angles.

Like I said, I am not a engineer, just interested in it. Maybe a different material, just don't know.

 

[DaveTooley]

There are physical limitations to every system.

 

[gunsandgunsmithing]

I am not a engineer, and I do understand the cost involved in some of this. Why not make the bolt head a little larger and the barrel extension a little thinner. Seems like it could open up to even more cartridges. The barrel extension/bolt head I guess can't be made much larger on the AR-15 platform. I also know rounded corners seem to work better than the sharp angles.

Like I said, I am not a engineer, just interested in it. Maybe a different material, just don't know.

It's been done. But keep in mind, you are fighting physics here. A std extension is already only about .0625 thick at the thinnest place, IIRC, so there's not much room to open it without creating a new weakest link. Bottom line is, smaller case heads produce less bolt thrust as do lower pressures. Some people are simply pushing the capabilities of the ar15 platform, for reliability. Frankly, it was bound to happen with the 6ARC. It was already happening with 223/556 stuff but the increased bolt thrust with the larger case id is exasperating and exposing the inherent weakness of the ar platform. Great gun..but we are at it's limits without significant re-engineering. In no way am I badmouthing an ar15. It's just that we are at its limit with a larger case head. It's not that the bolt head is thinner, its that there is increased bolt thrust any time the cartridge case id is increased. But to your credit, you do seem to realize that rather than blaming the bolt design. The platform just has limitations and it was a bit over built for its original 556 cartridge but not so for the larger case head of the grendel based cartridges. I don't see a reliable solution other than to accept it for what it is or to step up the bore size, ie a 30 cal, due to lower pressures with the same case and bullet weight to make the same speed with...at the cost of ballistics.

The problem is not the cartridge and it's not the gun. It's this cartridge, in this gun, due to the inherently higher bolt thrust. We can not and should not expect the same cartridge max psi when the bolt thrust is increased with a fatter case. It is what it is.That said, the ar10 can now be aquired in a gun that's about the same weight and you won't have this same problem in that platform and at these pressures.

 

[gunsandgunsmithing]

There are physical limitations to every system.

Exactly right! The ar15 is a great platform for which it was designed.

 

[gunsandgunsmithing]

Exactly right! The ar15 is a great platform for which it was designed.

Hornady created a monster here and they shouldve known the pressure limitations of the platform left little margin for error for safe reliability with it and the arc. Again, it is what it is. They advertised velocity numbers that are too near what the gun can stand reliably and they had to know handloaders would try to reach those numbers and more, especially with a cartridge that likes pressure to perform to its potential.

 

[DaveTooley]

There are design changes to this system that can help. Don't ask that's all I can say.

 

[troupe]

Great explanation on the bolt thrust. Thanks for taking the time on it. So, if the plan to shoot this caliber, and I do plan on shooting mine some, keep an extra bolt or two. It is at it's max, and sounds like it is beyond the max. I do hope our men and women that may be using it in harms way has some options, because they may not have the time to swap out a bolt when it goes down.

 

[gunsandgunsmithing]

Great explanation on the bolt thrust. Thanks for taking the time on it. So, if the plan to shoot this caliber, and I do plan on shooting mine some, keep an extra bolt or two. It is at it's max, and sounds like it is beyond the max. I do hope our men and women that may be using it in harms way has some options, because they may not have the time to swap out a bolt when it goes down.

Just load it accordingly, but yes, IMHO, hornady data is very borderline high for the platform, as is the factory ammo. But lots to that...as some powders might make their speeds without creating problems. Remember the 204 and the 5.7x28 that Hornady specifically said that their speeds were attained with proprietary powders?

FWIW, the same weight bullet, at the same velocity, but in a larger bore will create lower pressures and lower bolt thrust...all else equal. Maybe hornady should've done a 6.5 ARC. Lol!
..Or maybe just lowered the velocities and pressures just a tad. If you handload for it, you can do just that. Velocity sells guns and ammo. Roy Weatherby made a living at that but he had incredibly long freebores to help build in some cushion while getting big numbers on a chrony.

 

[moorepower]

I am up to 3-4 loadings at factory gas gun max book loads with the brass looking good yet. While quick load might be close, it won't replace a pressure barrel for accurate 52000 psi loads. There's nothing wrong with the round running the designed psi in an AR. When you start changing bullets, primers and powders, because "it should work", don't blame the round because your bolt fails. The JP bolt is a big step up from most other bolts.

 

[David Milisock]

Just load it accordingly, but yes, IMHO, hornady data is very borderline high for the platform, as is the factory ammo. But lots to that...as some powders might make their speeds without creating problems. Remember the 204 and the 5.7x28 that Hornady specifically said that their speeds were attained with proprietary powders?

FWIW, the same weight bullet, at the same velocity, but in a larger bore will create lower pressures and lower bolt thrust...all else equal. Maybe hornady should've done a 6.5 ARC. Lol!
..Or maybe just lowered the velocities and pressures just a tad. If you handload for it, you can do just that. Velocity sells guns and ammo. Roy Weatherby made a living at that but he had incredibly long freebores to help build in some cushion while getting big numbers on a chrony.

Certainly a system that is pushing the limits. In the case of the OP these limits seem to have been exceeded.

I only plan on using Leverevoltion powder, it's what Hornady used in designing the system and heavy projectiles. A loader needs to be diligent, work up slow and load in lots, reworking for any change in the next lot.

These parameters are not new, I've loaded 65,000PSI cases in my varmint rig for 40 years, that's HOT. I load 44,000 PSI handgun loads in plastic handguns, makes you diligent. Keep records on your powder lots, your bullet lot, primers and brass.

DO NOT EXCEED published loads for this system, if you want to juice the 6MM ARC, use a bolt gun. Or get real inventive with a custom gas gun.

With all this said the military would have been smart to build an AR15 length heavy action rifle to use the 6MM ARC at 63,000 PSI.

 

[David Milisock]

I am up to 3-4 loadings at factory gas gun max book loads with the brass looking good yet. While quick load might be close, it won't replace a pressure barrel for accurate 52000 psi loads. There's nothing wrong with the round running the designed psi in an AR. When you start changing bullets, primers and powders, because "it should work", don't blame the round because your bolt fails. The JP bolt is a big step up from most other bolts.

I agree, my only caution is the same for any cartridge that has SAAMI specs that are at maximum safe for the design. BEWARE, there is little to no headroom!

A 65,000 PSI 6MM Remington has ZERO headroom, a 55,000 PSI 6.5x55 in a modern Remington 700 action has plenty of headroom. A 52,000 PSI 6MM ARC gas gun has no headroom!

 

[lb-ft]

Before the advent of the ARC, I believe I read where some individuals enamored with the 6.5 Grendel in the AR15 platform were changing bolts for new every XXX rounds.

Perhaps this is a way forward for you. It works in aircraft, race cars, expensive machinery.

 

[David Milisock]

Before the advent of the ARC, I believe I read where some individuals enamored with the 6.5 Grendel in the AR15 platform were changing bolts for new every XXX rounds.

Perhaps this is a way forward for you. It works in aircraft, race cars, expensive machinery.

I gave a good deal of thought about the concept of the 6MM ARC before making the leap. The reason I did that was because while the modern concept of gunsmithing with the AR 15 platform has produced some great rifles it also has produced a large amount of trash from hell. In my opinion the used AR market has been ruined.

I did as much research as best I could, I had an advantage as my gunsmith knows the people at Aero Precision. Aero markets complete uppers, they also make matching lowers. They partner with manufacturers and assemble and test the uppers themselves.

If you buy receiver from A, bolt carrier from B, bolt from C, (you get the point) you bear the responsibility of quality not only of each component but of the resulting assembly. I didn't think that was a good idea with a design like a 6MM ARC that pushes the limits.

I bought a 16" upper, the Aero M4E lower, some A2 sights, a National Match trigger, buffer and the rest of the setup were recommended by a gunsmith I know. The upper comes fully assembled and tested, I had the smith assemble the lower, cost me some Turkey Hill, ice cream. I also bought a few magazines and as brass is over a buck a piece I got a brass catcher.

My goal was a rifle that handles as good as the M2 I carried in close and had extended range capability over my AR 15 A2 with one projectile weight. MY A2 is a 500 yard rifle in light wind but only with 69 grain Hornady, in wind I save the ammo, in close the A2 is a bit long. My Aero M4E is really good in close with rapid fire and extended range work is much better than my A2. I catch the brass and it looks fine. My guesstimate is to end up carrying the 103 grain Hornady ELDX as the only projectile. I'm more confident in the assembly for reloading than I would be if it were a mix master of my making.