6 arc bolt troubles

[uncle fester]

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.

You cannot duplicate the 6 ARC performance in a lightweight carbine in an AR10 lightweight carbine.

 

[JSH]

99.9% of the problems I see with AR’s are NOT 5.56/223. I gave up all thoughts of any other cartridges.

 

[David Milisock]

You cannot duplicate the 6 ARC performance in a lightweight carbine in an AR10 lightweight carbine.

I'm of the opinion that the Grendel and ARC need an intermediate AR platform action, same length as the current 15 but a bit larger in diameter to all the 63,000 PSI bolt rifle limit.

 

[Laurie]

An interesting free Handloader magazine piece and video here using an 18-in barrel AR and 26-inch barrel bolt gun and appropriate loads for each, for those who haven't already come across them.

https://www.handloadermagazine.com/6mm-arc (print)

https://www.handloadermagazine.com/handloading-and-shooting-the-6mm-arc (video)

The difference between the results from the two platforms is quite something even if a good chunk comes from the eight-inch barrel length variation.

As we're too irresponsible to be trusted with semi-auto platforms of any type (.22 rimfires aside) in the UK, I'm an interested outsider on the main thrust of this topic, but do expect to rebarrel a 6.5mm Grendel Howa Mini Oryx to the ARC at some future date. Whilst the 6.5 is impressive in the little Howa, I expect a good performance upgrade from the ARC (allied to a few extra inches of barrel length in the change).

 

[David Milisock]

99.9% of the problems I see with AR’s are NOT 5.56/223. I gave up all thoughts of any other cartridges.

I would disagree with that, the 6.8SPC, 300 Blackout seem seem fine. If I had a 300 in my house I'd have color coded magazines.

What I see is multifaceted, one, the AR has become the most prolific self loading action ever sold, so there are sure to be more in the shops being repaired.

Two, the modular nature of the platform not only has manufacturers making modified versions, many users are making modifications, some good some bad.

Three, my gunsmith of 40 years repairs many ARC15's, most by a very large margin are chambered 223, 5.56, 223 Wylde and a few 300 Blackout.

Four, according to my smith 99.9% of the repairs are damage due to user error. A huge amount of the assembly business he gets are for customer requested customizations.

 

[David Milisock]

An interesting free Handloader magazine piece and video here using an 18-in barrel AR and 26-inch barrel bolt gun and appropriate loads for each, for those who haven't already come across them.

https://www.handloadermagazine.com/6mm-arc (print)

https://www.handloadermagazine.com/handloading-and-shooting-the-6mm-arc (video)

The difference between the results from the two platforms is quite something even if a good chunk comes from the eight-inch barrel length variation.

As we're too irresponsible to be trusted with semi-auto platforms of any type (.22 rimfires aside) in the UK, I'm an interested outsider on the main thrust of this topic, but do expect to rebarrel a 6.5mm Grendel Howa Mini Oryx to the ARC at some future date. Whilst the 6.5 is impressive in the little Howa, I expect a good performance upgrade from the ARC (allied to a few extra inches of barrel length in the change).

Here in Pennsylvania bolt guys are getting nice results with the ARC very near 243 Winchester velocities with the same bullet weights. All from small actions, 22 to 26" barrels. SAAMI lists a higher limit for the bolt. It makes a good medium game rifle and a fantastic youth rifle.

I suggest if you build custom that you send an assembled dummy cartridge to your smith. Modify the throat and or magazine if required.

 

[JSH]

I would disagree with that, the 6.8SPC, 300 Blackout seem seem fine. If I had a 300 in my house I'd have color coded magazines.

What I see is multifaceted, one, the AR has become the most prolific self loading action ever sold, so there are sure to be more in the shops being repaired.

Two, the modular nature of the platform not only has manufacturers making modified versions, many users are making modifications, some good some bad.

Three, my gunsmith of 40 years repairs many ARC15's, most by a very large margin are chambered 223, 5.56, 223 Wylde and a few 300 Blackout.

Four, according to my smith 99.9% of the repairs are damage due to user error. A huge amount of the assembly business he gets are for customer requested customizations.

When I mention issues, I refer to some of the “new” pressed into service cartridges.
The AR platform has turned into tinker toys for adults. Most with good intentions but poor knowledge by so many backyard builders.

They can be made to run with several cartridges and do it well. Yet some build and leave out or cheap out on major parts. The modifications at times are some what questionable.

The mention of working on a fair number of the 300’s, wyldes and 223/5.56, probably because they see more use than the others.

What I know about AR’s will fit in a thimble compared to many. I have a couple in 223/5.56 and they run fine and are dependable. At the range or at the shop I see soooo many having issues, weather it be operator error or cartridge. A lot of times poor maintenance is a major issue.

Sorry to the OP for the derail.

 

[gunsandgunsmithing]

A 95

You cannot duplicate the 6 ARC performance in a lightweight carbine in an AR10 lightweight carbine.

I'm missing something here. What won't a 243 or 6 creedmoore do, in terms of performance and what is the limiting factor(s) that make you say that, specifically? Don't get me wrong ..I love the highly efficient small cartridges, like the arc so I get that part. Just not sure what you're specifically saying the arc can do that those can't do. Fwiw, I do agree that a redesigned ar that can reliably handle more bolt thrust would be nice. Maybe someone will step up and do something in that regard. Not sure that it could be limited to the upper only but maybe.

 

[uncle fester]

A 95

I'm missing something here. What won't a 243 or 6 creedmoore do, in terms of performance and what is the limiting factor(s) that make you say that, specifically? Don't get me wrong ..I love the highly efficient small cartridges, like the arc so I get that part. Just not sure what you're specifically saying the arc can do that those can't do. Fwiw, I do agree that a redesigned ar that can reliably handle more bolt thrust would be nice. Maybe someone will step up and do something in that regard. Not sure that it could be limited to the upper only but maybe.

The 6mm CM and 243 are both (essentially) built around 308 sized cartridges and pressures. As a result, the are faster than the 6mm ARC if you push them. But, you need a traditional sized bolt action or an AR 10 sized action which means a heavier overall rifle.

The entire reason the 6ARC was invented to approximate .308 performance in a AR15 platform. The military has tons of AR 15 lowers they can use with a 6ARC upper. They are much lighter systems than any AR10 rifle would be. Judged by that standard, it succeeded. They took the AR 15 platform as far as it can go.

But, the weak point in the system is the AR15 bolt is not as strong as it could be. I view it as a part likely to fail over time and plan on replacing it regularly.

 

[gunsandgunsmithing]

The 6mm CM and 243 are both (essentially) built around 308 sized cartridges and pressures. As a result, the are faster than the 6mm ARC if you push them. But, you need a traditional sized bolt action or an AR 10 sized action which means a heavier overall rifle.

The entire reason the 6ARC was invented to approximate .308 performance in a AR15 platform. The military has tons of AR 15 lowers they can use with a 6ARC upper. They are much lighter systems than any AR10 rifle would be. Judged by that standard, it succeeded. They took the AR 15 platform as far as it can go.

But, the weak point in the system is the AR15 bolt is not as strong as it could be. I view it as a part likely to fail over time and plan on replacing it regularly.

There are some pretty light ar10s out now. Granted, if weight is the objective, I'm sure you could do much of the same things with an ar15 and be lighter yet. Here's a link to an ar10 under 6lbs.

ROAM R-10 – World's Lightest Production AR-10 Now Available

'World's lightest production AR-10's' - that's what a company called Roam claims when describing their new R-10 rifles. The lightest of Roam R-10 line of rifles is the R-10 Red River Ultralight which being chambered in .308 Win and having a 16' pencil barrel, weighs in at 5.73 lbs.

www.thefirearmblog.com

 

[uncle fester]

There are some pretty light ar10s out now. Granted, if weight is the objective, I'm sure you could do much of the same things with an ar15 and be lighter yet. Here's a link to an ar10 under 6lbs.

ROAM R-10 – World's Lightest Production AR-10 Now Available

'World's lightest production AR-10's' - that's what a company called Roam claims when describing their new R-10 rifles. The lightest of Roam R-10 line of rifles is the R-10 Red River Ultralight which being chambered in .308 Win and having a 16' pencil barrel, weighs in at 5.73 lbs.

www.thefirearmblog.com

Fair point. But, those things are incredibly expensive and I doubt they would have done well in a military environment.

They look cool and I definitely would love to try one.

 

[gunsandgunsmithing]

Fair point. But, those things are incredibly expensive and I doubt they would have done well in a military environment.

They look cool and I definitely would love to try one.

I agree that its a nice looking rifle. I din't even look at the price. There is this one at 7.6lbs.

Login

 

[Laurie]

But, the weak point in the system is the AR15 bolt is not as strong as it could be. I view it as a part likely to fail over time and plan on replacing it regularly.

I'm sure I read somewhere some good time ago that the US Army concluded that 7.62X39mm AR-15s were likely to suffer bolt problems at around the 9,000 round mark and would need earlier programmed prophylactic replacement - an unwanted complication of the sort armies can well do without. SAAMI gives the 7.62X39 an MAP of just over 45,000 psi, although I suspect much eastern European military ammo is loaded up to higher pressures. European CIP with its different pressure measurement system shows it at just over 51,000 psi, very much in line with the ARC. And of course, 7.62X39 is the parent of the Grendel / ARC so has the same case-head. Either way, that 9,000 round bolt life drops dramatically in this (ARC) chambering. Of course, today's bolts and materials may be a lot more stress resistant than was the case at the time of this evaluation - my knowledge of metal alloy strengths and recent progress is akin to zero!

To me, this suggests that any move to a cartridge based on the old M43 requires a completely redesigned and much more robust bolt / rifle upper assembly. Throw in new magazines and other odds and sods and it becomes a very expensive exercise indeed in military circles.

Interestingly though is the US firearms industry adopting a cartridge and selling firearms and factory ammunition for civilian use that could produce a major structural failure within the expected lifetime of a second barrel - forget overloaded handloads and gash poor quality DIY or second-rate workshop builds. Perhaps I have an 'old-fashioned' view here, but I think of barrels as being disposable commodities to be replaced when shot out, but the rest of the piece goes on virtually indefinitely for all but a tiny number with usage levels far beyond the norm. Most of us wouldn't accept car manufacturers selling us products where the drivetrain is likely to suffer catastrophic collapse half way through the life of the second set of tyres. Still, as @JSH says in an earlier post, maybe the number of 223/5.56 AR failures at a county gunsmith level is already pretty high.

Just some thoughts from an ignorant Brit who's not allowed to own these attractive and fascinating rifles, or at any rate not with a gas-operating system. (I owned a manual 'straight-pull' AR-15 in 223 Wylde, rebarreled later to 6.8 SPC that was a very satisfying and accurate piece. One thing though about these rifles in this form is that there is a very strong disincentive to use very heavy loads even in 223 given the effort required to start bolt opening / case extraction without gas assistance. For this reason, the 223 remains the norm here as the Grendel and similar fatter-case designs cannot be loaded up heavily enough to make use of their superior ballistics. 224 Valkyrie seems to be as big as you can comfortably go.)

 

[Bill K]

All types of calibers are or have been put into the Armalite uppers and they work just fine, when set up to feed and lock up properly.
When lugs start to break or bolt carriers start to get loose, break, etc. the shooters loads are too hot and pressures are way to high.
Back off on those loads. Play safe and sane.

 

[Herzo]

While apparently a persona non grata among several of the websites, I purchased a 6mm predator from a vendor who produced a bolt with lugs with a trapezoidal configuration that were approximately 30% wider at the base. I have not seen that he still offers it, but it certainly seemed to be a solution go the problem. The barrel adapter was modified to handle the design, but was supposed go be compatible with the standard bolt desygn.

 

[David Milisock]

A 95

I'm missing something here. What won't a 243 or 6 creedmoore do, in terms of performance and what is the limiting factor(s) that make you say that, specifically? Don't get me wrong ..I love the highly efficient small cartridges, like the arc so I get that part. Just not sure what you're specifically saying the arc can do that those can't do. Fwiw, I do agree that a redesigned ar that can reliably handle more bolt thrust would be nice. Maybe someone will step up and do something in that regard. Not sure that it could be limited to the upper only but maybe.

I am of the opinion that AR 10 platform rifles and cartridges make fine specialized firearms. However poor choices as general issues rifles.

My 8 pound loaded rifle in 6MM ARC is really fast acting in close, (recoil is amazing light), rate of tactically accurate semi-auto fire is very high, recovery time and target acquisition is better than many modern pistol caliber carbines. Based on my experience it is quite possibly the best CQB rifle I've ever used. It hits level 4 plates hard enough to cause serious soft tissue damages in close. A 108 grain projectile at over 2,500 FPS. I've made 600 yard hits in 15 MPH winds and higher.

To my knowledge no AR 10 based cartridge can do all this in terms of all around performance, especially in close with a high rate of accurate fire. In my opinion in close is where the crap gets sporty! When the work is far its generally slower and more accurate. To make a fast acting rifle that can work in close and go out to 800 yards accurately is very hard. It's taken over 50 years to get one for general commercial sale.

The OP has shown the issue with the 6MM ARC and 6.6 Grendel when seeking advanced performance. However, the loads used by the OP were hotter then recommended and a light bolt carrier may have had the camming action on the bolt at a time where pressure was higher then as designed.

At least one special operations group has acquired their first shipment of AR 15's in 6MM ARC, the problem being, getting information about the reliability in combat will be difficult at best. I have zero concern as my use is as a civilian and I've been loading for auto loaders for well over 40 years.

 

[Laurie]

To my knowledge no AR 10 based cartridge can do all this in terms of all around performance, especially in close with a high rate of accurate fire.

In the AR-15 v AR-10 debate, people seem to have forgotten that David Tubb went down the big black rifle route some considerable years ago and decided it was a wrong turn. That's why he designed the Tubb 2K bolt-rifle for XTC type shooting because he'd given up on trying to make 260 Rem class AR-10s competitive in rapid-fire three position competition. If the 260 Rem plus AR-10 combination lacks that edge in this sort of competition / use in the hands of a true master, then the smaller, lighter and low-recoil 6 / 6.5mm 15s are going to have an even greater advantage in CQB and short distance competition disciplines as you say. If today's 6mm ARC AR-15 had been available to David Tubb 20 odd years ago, I wonder if he'd have tried that. (Still, the greater performance consistency of an optimised bolt-gun platform may well still win out anyway.)

Alright, I know that the T2K and 6XC combination is much more than a short-range rapid-fire XTC tool with its thousand yards precision shooting capability, but the 6ARC AR-15 has to be an interesting contender in up to 600 yards type events. Keeping track of round count and preventative bolt replacement shouldn't be an issue for such users either.

 

[SKWERLZ]

Someone needs to look at what sig did on the bolt for the new us military rifle and apply it to the AR-15.

 

[David Milisock]

In the AR-15 v AR-10 debate, people seem to have forgotten that David Tubb went down the big black rifle route some considerable years ago and decided it was a wrong turn. That's why he designed the Tubb 2K bolt-rifle for XTC type shooting because he'd given up on trying to make 260 Rem class AR-10s competitive in rapid-fire three position competition. If the 260 Rem plus AR-10 combination lacks that edge in this sort of competition / use in the hands of a true master, then the smaller, lighter and low-recoil 6 / 6.5mm 15s are going to have an even greater advantage in CQB and short distance competition disciplines as you say. If today's 6mm ARC AR-15 had been available to David Tubb 20 odd years ago, I wonder if he'd have tried that. (Still, the greater performance consistency of an optimised bolt-gun platform may well still win out anyway.)

Alright, I know that the T2K and 6XC combination is much more than a short-range rapid-fire XTC tool with its thousand yards precision shooting capability, but the 6ARC AR-15 has to be an interesting contender in up to 600 yards type events. Keeping track of round count and preventative bolt replacement shouldn't be an issue for such users either.

David Tubb, holy cow what a rifleman!

 

[David Milisock]

Someone needs to look at what sig did on the bolt for the new us military rifle and apply it to the AR-15.

I agree, some development on the receiver and bolt.

Keep a general issue rifle light, fast, as hard hitting as possible and accurate. Engaging from 200 to 800 yards is one aspect, engaging in close, end of the muzzle, across the room, very rapidly becomes a very serious situation. Todays rifleman needs both abilities in an equal measure situation as possible.