Design of bolts for ARs

[cory]

The Elk hunting story illustrates what happens when you are able to practice with a low-recoiling cartridge that has good projectile selection with bullets that have high sectional densities. In this case, the guy ventured off to find an elk to shoot early in the morning, only to hear his wife make a shot, then he finds that she DRT'd it in his absence.

There are some people who are suggesting that we need more and more muzzle velocity, when the currently-available system is able to harvest game as documented above. These performance demands require a different bolt and extension to be made, so that one can chase the .260 Remington out of the AR15 platform, which you will never reach unless you increase case capacity and pressure, since the .260 Rem is a 60,000 psi SAAMI-rated cartridge.

Even my heavy 22" target-barreled .260 Rem AR10 can be stiff on the shoulder. I wonder what other parts would start to give if I stuffed that much pressure into an AR15, even if the bolt and extension were able to handle it. There are some engineering considerations that need to be accounted for when asking for .260 Rem performance in the AR15 receiver set. The DPMS LR-308 bolt isn't really adequate at dealing with .260 Rem pressures, since it has a rather large firing pin hole.

I think most of us here in the discussion of pushing the Grendel to higher MVs recognize that the Grendel will never get .260 performance.

I for one just want to see the Grendel fully utilized. I believe the AR15 is selling the round short.

[LRRPF52]

An AR12, at least as I envision it would sacrifice little in parts commonality with what's on the market for ARs now.

There's no reason an AR12 shouldn't utilize the same LPK. Maybe, just maybe it'll need a longer mag release latch.

I see no reason it shouldn't utilize the AR15 carrier. Therefore it should utilize the AR15 charging handle, unless you want to redesign it to utilize a slick side charging handle.

It should be able to utilize AR15 rifle and carbine buffer system. However, I think it's likely that it would greatly benefit from an VLTOR midlength buffer system or something similar.

Essentially a weapon could be designed around the Grendel (& the 6.8 to make it a more profitable investment) that would only require a specifically built lower, upper, bolt, & extension. Oh and magazines.

AR15 and AR10 mag catches are the same. The main lower parts differences are the pivot and take-down pins, then bolt catch. The question is, what are we achieving if we make this slightly larger receiver set? It would certainly be welcomed by some of the people who like to wildcat, but most shooters will not be capable of or care to exploit the extra 150-200fps. Then we move into the business decision aspect of it.

I actually think the market is only going to increase for different calibers, as hunting with AR15's becomes more and more the norm. When the market will bear regular sales of such a receiver set and series of cartridges, along with mitigating the safety precautions of ammunition commonality, then we'll start to see larger case capacities that are somewhere between a Grendel and a .308, as well as some of the short fatties that hold more capacity than a .308, moving into 6.5-06 performance territory.

I think most of us here in the discussion of pushing the Grendel to higher MVs recognize that the Grendel will never get .260 performance.

I for one just want to see the Grendel fully utilized. I believe the AR15 is selling the round short.

Tell that to my 16" Grendel with the 123gr. If most shooters aren't capable of exploiting the performance edge the Grendel has, why throw out the AR15/Grendel combo when it works so well? Without the AR15, there would be no Grendel.

I think a lot of this is driven by an arbitrary desire to just have more muzzle velocity, not fully realizing what you already have. I was guilty of that until I took it out to 1200yds. I can't say this enough; it honestly surprised me.

[Tedward]

I know there are some that want more and more but I think the Grendel in the AR-15 is great and do agree with the low recoil. And yes, you want more beef, get the AR-10 and go 260 or something.

i just wanted to know about the Grendel Extractor and barrel extension issue if there is one. I don't think there is. If someone goes for more and more hotter loads, yes they will cause issues or premature failures. I like the round as it is and feel to just drop back on the load to make it reliable, not get that extra 5fps. Just want to make sure I'm not missing something using the standard safe, 50k/55k psi round. If the 60k psi is the breaking point or early failure loads.

The AR-10 and AR-12.25 is a different subject as far as I am concerned.

thanks for the info

[SHORT-N-SASSY]

. . . I for one just want to see the Grendel fully utilized. I believe the AR15 is selling the round short.

+1

[LRRPF52]

+1

What are your experiences with this cartridge in the AR15? There are tons of people who are stacking their freezers with meat every hunting season, and taking animals that people would have said were unethical if you told them that you would be hunting elk, caribou, feral hogs, mule deer with an AR15.

I think the AR15/Grendel is a wonderful match, based on shooting and loading for it since 2009. When I take new shooters who have never shot at distance, put them behind a little 16" Grendel, and they are ringing steel at 300-600yds as if it was normally easy (in full value winds at 12-20mph speeds), it clearly is a highly-capable system that takes the AR15 to performance levels that are closer to the .308 Winchester, with less wind drift, 50% less recoil, and 85% of the .308's energy at distance that can clearly be witnessed when shooting steel.

[cory]

LRPF52 I didn't realize that about the mag cathces, as I've yet to own or build an AR10. I don't believe you'd need different takedown or pivot pins from the AR15 utilizes. I could be wrong, but I don't think that'd be a failure point if we left it at the current spec.

who said anything about throwing out the Grendel/AR15 combo. You'd still have that option.

It'd be like the forged vs billet choice. You can still go cheaper forged option that's more than adequate or you can go the more expensive billet route and get a better fit. Just an analogy.

We certainly don't NEED an AR12 that fully utilizes the Grendel, but can tell me you wouldn't be one of the first to build one if a company offered it at a reasonable price? I'd have one preordered!

[SHORT-N-SASSY]

. . . Without the AR15, there would be no Grendel. . . .

I think it's fair to state that if the 6.5 Grendel was initially designed to work with a robust platform, such as the Remington R-15 30AR or the ARPerformance Big Bolt, instead of "shoehorning" it into the weaker AR-15, we wouldn't be having this Discussion.

[Tedward]

Cory, We all understand making another AR like a AR-12 would cost millions and the propitiatory parts from the upper receiver itself (longer to fit the longer lower due to the longer magwell), lower receiver and mags. You are right that it would let you use a longer COL. Then the next guy would want a AR-13 so the COL can be 2.45 or 2.55. It is a never ending of wants.

When I hit the lottery I'll make one of each and you will be first on the list. Or just figure out how to lighten up the AR-10. I do like the small compact AR-15 Grendel and if it were to go to an AR-12, the recoil would increase with heavier bullets and more powder, then your back to breaking bolts and then need an AR-12 extension and bolt. One change leads to another and another and proprietary product.

anyhow we can all dream and mine is to hit the Lottery.

[cory]

Tedward, yes???

Hornady lists the max col for the 308 at 2.81". I assuming that's the max col for the AR10. Either way at a COL of 2.7" or greater then your well into AR10 territory. Not to mention that much COL would be useless to the Grendel and likely useless to the 6.8 too.

Once an AR12 is built your max pressure would be derived per the case limitations, which I'm assuming would be around 58ksi. You're right in recoil would increase for heavier projectiles. There's nothing to stop you from loading to 52ksi if lower recoil is what you seek. At 56-57ksi we're looking at an ~5% recoil increase. I don't think it'd be that significant.

To design and build a new FORGED AR, yes it would costs millions. A billet AR12 could be machined in existing CNC machines once the design is CADed.

[Tedward]

I think it's fair to state that if the 6.5 Grendel was initially designed to work with a robust platform, such as the Remington R-15 30AR or the ARPerformance Big Bolt, instead of "shoehorning" it into the weaker AR-15, we wouldn't be having this Discussion.

Thats called the 260 or any 6.5 in a larger AR-10 case.. I think that's what LRRPF52 is saying too.

[SHORT-N-SASSY]

Thats called the 260 or any 6.5 in a larger AR-10 case.. I think that's what LRRPF52 is saying too.

No, Tedward, I'm referring to the cartridge that we all know and love, the 6.5 Grendel, but loaded to the safe limits of its case --- possible in the Remington R-15 30 AR and the ARPerformance Big Bolt, but proven not possible in the standard AR-15 Bolt-Barrel Extension; i.e., without experiencing some degree of Bolt failure.

[cory]

Ted see my edited post #39

[LRRPF52]

If someone wants to invest in a slightly larger receiver set, larger custom-made bolt and extension, for that extra 150-200fps, you could start with the Six8 receivers, find a way to source some of the Six8 PMAGs and different Bolt Carrier, and have a custom set-up built. Cost would be pretty significant.

[LRRPF52]

No, Tedward, I'm referring to the cartridge that we all know and love, the 6.5 Grendel, but loaded to the safe limits of its case --- possible in the Remington R-15 30 AR and the ARPerformance Big Bolt, but proven not possible in the standard AR-15 Bolt-Barrel Extension; i.e., without experiencing some degree of Bolt failure.

We don't see bolt failure if bolts are made from certain alloys and heat-treated correctly. Many of us have significant round counts on our bolts with no issues, and many of us have fired scores of higher-pressure loads when we were doing development, not that the loads are mass-produced, but you find your max when developing loads. Bwild97 has over 4000 rounds on his 24" Grendel, and he has done a lot of development pushing the edge.

Where people get into trouble is when the heat-treating isn't done just right on 8620, which will break even on .223 bolts. AA has a very stringent QC process for testing bolts, using superior alloys to 8620.

A lot of the bolts that have broken have been after-market 7.62x39 bolts made to lesser standards, and they have a reputation for breaking in the 7.62x39 AR's as well.

[Tedward]

Tedward, yes???

Hornady lists the max col for the 308 at 2.81". I assuming that's the max col for the AR10. Either way at a COL of 2.7" or greater then your well into AR10 territory. Not to mention that much COL would be useless to the Grendel and likely useless to the 6.8 too.

Once an AR12 is built your max pressure would be derived per the case limitations, which I'm assuming would be around 58ksi. You're right in recoil would increase for heavier projectiles. There's nothing to stop you from loading to 52ksi if lower recoil is what you seek. At 56-57ksi we're looking at an ~5% recoil increase. I don't think it'd be that significant.

To design and build a new FORGED AR, yes it would costs millions. A billet AR12 could be machined in existing CNC machines once the design is CADed.

Cory,
though about it and yes making a billet might be less costly but as Mark, PF, told me last week we think it is easy to change the CAD drawing and the length, that is easy It's all the programming to link all the cutters, milling and EVRYTHING behind the scene. So, ya it could be made but made me think past the Upper and lower.
I think a longer carrier would then be needed too for the longer upper, sort of in between the AR-15 and AR-10. As we all know, its all about money...Also I realized I put the wrong hypothetical OAL on my last post, I changed it.

Anyhow, heading out and will catch up later. Let me know how it comes out. I'll take one to test too.