6 arc bolt troubles

[David Milisock]

To expand a bit on this 6MM ARC, 6.5 Grendel bolt limitation. A limitation, that is what it is nothing more. Hand loaders WILL NOT be getting a performance boost in the AR15 platform, PERIOD! I didn't buy mine thinking thatvI would get a boost, in fact I knew that I'd have to identically match the factory loads.

The platform and cartridge were designed at peak platform capability. We've seen this before especially with gas operated systems.

I hope the process works out like it has in the past. The Garand went through growing pains but it was over designed for the 06 in the first place, especiallythe M2 Ball.

Here we have a peak design for an old (60 years old) platform. If this cartridge works out and the military wants improved performance from this cartridge it will need a new platform, an AR 20 or something, it will not be the AR 15. However there is room for improvement, a near 8 pound rifle with a 108 cartridge nearing 3,000 FPS, not a bad idea. Even the short 14" barrels could be near 2,700 FPS.

 

[243Mendoza]

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.

Yup, I was thinking the same design with the added feature of a fillet between lug and body to alleviate stress concentration.

 

[243Mendoza]

To... However there is room for improvement, a near 8 pound rifle with a 108 cartridge nearing 3,000 FPS, not a bad idea. Even the short 14" barrels could be near 2,700 FPS.

not going to happen...safely with the AR15!

 

[243Mendoza]

David Tubb, holy cow what a rifleman!

That SIG rifle has more in common with the AR10 than the AR15.

 

[David Milisock]

not going to happen...safely with the AR15!

They'll need a redesign for sure.

 

[David Milisock]

That SIG rifle has more in common with the AR10 than the AR15.

Too damn heavy!

 

[constructor]

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.

Yes but a DPMS G2 weighs only 8 oz more than a similar built 6mmARC. I have both 1/2lb difference. In addition The 6mmARC is a copy of several other cartridges or cartridge that has more than a few names since 2005 or 6. 6mmAR, 243LBC, 6mm Predator, 6mm Banshee, 6mm Grendel with the only difference being the ARC was made apx .020" shorter.

 

[constructor]

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.

I posted a photo of the first large bolt I machined in 2007 here in these forums. Shortly after a guy named Kramer from Vegas filed for a patent and around 2015 they gave him a patent so if anyone tries to make a bolt with a larger diameter head Kramer will sue. He tried to sue me until I proved he likely got the idea from the photo I posted here 3 months before he filed for the patent. Since then I have retired and no longer make the big bolts.

 

[gunsandgunsmithing]

Yes but a DPMS G2 weighs only 8 oz more than a similar built 6mmARC. I have both 1/2lb difference. In addition The 6mmARC is a copy of several other cartridges or cartridge that has more than a few names since 2005 or 6. 6mmAR, 243LBC, 6mm Predator, 6mm Banshee, 6mm Grendel with the only difference being the ARC was made apx .020" shorter.

Thankfully too, I'm not worried about the best of the best battle rifle for personal use. 8 ounces won't mean I live or die..I hope. And the same goes for bolt life. I'm not currently depending om it to last xxx number of rounds to save my life, either. Frankly, I laugh at people who claim they have to have this or that because the military says so. In my game, a torso is like hitting the whole dang target! I wanna hit that little bitty dot that's about 20x smaller than a fly!

 

[constructor]

Yup, I was thinking the same design with the added feature of a fillet between lug and body to alleviate stress concentration.

I made these from 2007 until 2017, they were strong enough to run a 6mmBRX at 65000psi for 10 years. Like someone above said the grendel guys don't like me much LOL. I told them in 2006 the bolt was a problem and the mags were problems but instead of acknowledging those problems and working on a fix they just said the bolts don't break and the mags feed fine so here we are 16 years later and the same problems still exist.
Standard 5.56 on the left, 6.8 Superbolt, Titan (7.62x39/Grendel), 750XD(308 diameter and the 800 series(308 diameter discontinued in 2015).

 

[gunsandgunsmithing]

I made these from 2007 until 2017, they were strong enough to run a 6mmBRX at 65000psi for 10 years.

Are they the ones on the right? I remember those and yes, they were a lot stronger. They had a radius between the lugs, as those appear to have. Are you still making any of those and the bbl extensions? From what I recall, they were the best and it's easy to see why.

 

[constructor]

Are they the ones on the right? I remember those and yes, they were a lot stronger. They had a radius between the lugs, as those appear to have. Are you still making any of those and the bbl extensions? From what I recall, they were the best and it's easy to see why.

The 800 series on the right were the first ones I made starting 2007. In 2015 Kramer tried to sue me and rather than paying an attorney $500 an hour to prove his patent invalid I just redesigned the bolt to the 750XD, second from the right. I had to redesign again so it would work with the 6mm and 224 Predator cartridges, the thinner lugs/grooves in the barrel extension guided the bullets higher so they entered the chamber instead of slamming into the breech. In 2017 I semi-retired and stopped machining all of the bolts.

 

[gunsandgunsmithing]

The 800 series on the right were the first ones I made starting 2007. In 2015 Kramer tried to sue me and rather than paying an attorney $500 an hour to prove his patent invalid I just redesigned the bolt to the 750XD, second from the right. I had to redesign again so it would work with the 6mm and 224 Predator cartridges, the thinner lugs/grooves in the barrel extension guided the bullets higher so they entered the chamber instead of slamming into the breech. In 2017 I semi-retired and stopped machining all of the bolts.

Thanks! Good looking design!

 

[constructor]

Since the topic came up I'll point out another potential problem.
Bolts should be case hardened/gas carburized to the required hardness. IF a company then sends them out for Melonite or Black Nitride as H&M calls it that will anneal the material that has already been gas carburized. The lugs will over time compress increasing headspace. Many on the forums disagree but there is an easy test. Buy a nomally hardened barrel extension and a Black Nitride treated barrel extension. Lay them on an anvil and strike them with an 8 lb sledge. The silver normally hardened extension will break or shatter. The melonite treated one will fold up before it splits down the sides, it is softer or you can take them to any place that does heat treating and have them tested for hardness, not just surface but also at the core. They told me Black Nitride wouldn't anneal the extension either but I tested them and found out it did.

 

[gunsandgunsmithing]

Since the topic came up I'll point out another potential problem.
Bolts should be case hardened/gas carburized to the required hardness. IF a company then sends them out for Melonite or Black Nitride as H&M calls it that will anneal the material that has already been gas carburized. The lugs will over time compress increasing headspace. Many on the forums disagree but there is an easy test. Buy a nomally hardened barrel extension and a Black Nitride treated barrel extension. Lay them on an anvil and strike them with an 8 lb sledge. The silver normally hardened extension will break or shatter. The melonite treated one will fold up before it splits down the sides, it is softer or you can take them to any place that does heat treating and have them tested for hardness, not just surface but also at the core. They told me Black Nitride wouldn't anneal the extension either but I tested them and found out it did.

Makes sense to me. IIRC, meloniting is done at about 1100°. Not sure about the time but that much heat will alter hardness quickly..especially on thinner parts.

 

[troupe]

Patents run out, feel like coming out of retirement. I for one would love to see it. I knew there was a better way and you done it and been there. Thanks for sharing it, that is amazing. What about the Mutant that CMMG had shown. So the change that you done still has better edges on the lugs and more meat around the case head. A matching extension and this would work in a standard AR bolt carrier. It is amazing.

 

[constructor]

Patents run out, feel like coming out of retirement. I for one would love to see it. I knew there was a better way and you done it and been there. Thanks for sharing it, that is amazing. What about the Mutant that CMMG had shown. So the change that you done still has better edges on the lugs and more meat around the case head. A matching extension and this would work in a standard AR bolt carrier. It is amazing.

The CMMG bolt is plenty strong enough from what I have seen.

 

[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 have scoured the internet and for the Grendel and the 6MM ARC on an AR 15 platform I have not found any instances when I couldn't find some error in reloading or how the rifle was built. Granted that some of those posting have poorly descriptive posts. In the case of the original post, excessive pressures with a fast for bullet weight powder and a light weight bolt carrier.

I will add that with these AR chambering I would only buy fully assembled uppers from a manufacturer, it's just my opinion but these chambering run CLOSE to the line. A mix master in my opinion has a greater chance of tolerances being a maximum and as is this case a light weight bolt carrier changing the timing and pressure when the bolt unlocks.

Only time will tell but I have zero concers for my rifle as of this time. However I would ask, what should be expected for bolt life shooting factory or equivalent loads in the 6.5 Grendel and 6MM ARC? I would expect 5,000+ rounds of general purpose use and 2,500 of competition as minimums with the current design.

 

[troupe]

I sure hope so. I for one won't be shooting mine that much, only because it will be a hunting rifle. Once I get a load worked up that I like, I will be lucky to shoot 30 rounds a year. I would love to see someone like constructor be able to offer there set up for future builds. His designs can make the little AR that much better. Especially for the military aspect of it where a broken bolt could be the difference between life and death. I like the big AR's, but they are just so heavy. I have a AR-10 know that I have not even shot in a 6.5 Creed.

 

[David Milisock]

I sure hope so. I for one won't be shooting mine that much, only because it will be a hunting rifle. Once I get a load worked up that I like, I will be lucky to shoot 30 rounds a year. I would love to see someone like constructor be able to offer there set up for future builds. His designs can make the little AR that much better. Especially for the military aspect of it where a broken bolt could be the difference between life and death. I like the big AR's, but they are just so heavy. I have a AR-10 know that I have not even shot in a 6.5 Creed.

I agree, a little change and the military can get a great rifle.