308 style bolt in a AR-15 rifle

[montana]

Looks like CMMG just made a 308 bolt for a AR-15 rifle. The Grendel round would be a great option for this rifle.

http://www.thefirearmblog.com/blog/2014/...47-mutant/

[Mountain Man]

I think it's a great idea if it allows us to run 60k psi in a gas gun, don't see a huge advantage otherwise. Definitely cool though.

[LRRPF52]

It's not an AR15. You can see that it is based off the CMMG Mk.3, which is an SR-25 based receiver set (very large, even larger than the 1955-1962 AR10).

The benefit of the 6.5 Grendel/AR15 combination is that you get a very lightweight, easy handling receiver set (and all after-market parts), with retained energy more like .308 Winchester, but with half the recoil.

On CMMG's behalf, I will say that the Mk.3 is one of the lighter SR25 variants, but I believe the SR25 based guns are basically obsolete now with the introduction of the Colt 901 and the DPMS/Freedom Group G2. There are some basic mechanical engineering challenges faced with making a .308 work with the AR15 receiver extension tube, which was one of the main goals Eugene Stoner and Reed Knight II had when developing the SR25, introduced in 1993. An AR10 rifle had not been in production since 1962 until that point in history, and they were counting on the furniture and as many components from the AR15 being compatible with the new rifle, which lowered barriers to entry for manufacturing/production.

If you wanted to build a 6.5 Grendel with an AR10 bolt, the Colt 901 would be the best route currently.

[montana]

Well that would explain it. It was hard to tell looking at pictures. A comparative size next to an AR-15 would have been better. Not as revolutionary as I first thought.

[LRRPF52]

After comparing images of the Mk.3 and this mutant, it appears that they have shortened the back of the receiver set, which means the BCG is shortened in the tail, while retaining the basic dimensions of the SR25 BCG, and making accomodations for the AK mag feed lips to have sufficient penetration/vertical location for proper feeding.

This could be the foundation for CMMG to answer the DPMS G2 and Colt 901, if they offer it in 7.62x51 NATO as well. I had a very strong feeling that the market would respond to the G2, and this appears to be the first major answer, only in 7.62x39. They probably have some inventory and dealer agreements to finish out Mk.3 production, but I can't see many reasons to continue the Mk.3 with this development.

Looking forward to seeing a new .308-based CMMG receiver set that is shorter, as that is the trend. My personal interest is in other calibers based off of the .308 case, not .308 Win., since I can get basically .308 practical performance from the 6.5 Grendel from within the AR15 package currently, making .308 pointless for me personally.

[mongoosesnipe]

When are we going to get over this push the grendel into a 260 thing, it is a fantastic cartridge as it is and is not really lacking without being pushed, in the same way that no mater what you do to a 308 it won't turn into a 300 win mag the grendel can't compete with a 260 or 6.5 creedmoore the case capacity just isn't there I have seen some data using a bolt gun with the cartridges seated extremely long in order to get some extra case capacity but from an auto that pushes the cartridge out of the ar15 and into the ar10 so why not just use an ar10 in 260 or creedmore because as stated there is no possible way to push the Grendel to those velocities no matter the platform so why do we keep re hashing this 60k psi grendel pipe dream it's not safe, nor is it even practicle from a gained performance standpoint

Sorry for the long diatribe and I don't mean to bash on any new product that thing seems pretty cool but it doesn't change the Grendel game and considering it does not appear that the upper would be compatible with standard ar lower and Grendel doesn't stack to feed well from an ak mag what's the point

[LRRPF52]

I found some more information about it, and it is in fact a shortened SR25 bolt carrier group:

The Mk47 MUTANT utilizes a unique bolt carrier group derived from the massive AR10 group, which aside from being shortened to a length of eight inches, retains all of the material on the bolt face for added durability and strength.

http://soldiersystems.net/



That must be the overall length of the entire BCG, not the carrier. For comparison:

SR-25 Bolt Carrier: 7 7/8"
AR15 Bolt Carrier: 6 11/16"
Colt 901 Bolt Carrier: 7 1/4"

[montana]

When are we going to get over this push the grendel into a 260 thing, it is a fantastic cartridge as it is and is not really lacking without being pushed, in the same way that no mater what you do to a 308 it won't turn into a 300 win mag the grendel can't compete with a 260 or 6.5 creedmoore the case capacity just isn't there I have seen some data using a bolt gun with the cartridges seated extremely long in order to get some extra case capacity but from an auto that pushes the cartridge out of the ar15 and into the ar10 so why not just use an ar10 in 260 or creedmore because as stated there is no possible way to push the Grendel to those velocities no matter the platform so why do we keep re hashing this 60k psi grendel pipe dream it's not safe, nor is it even practicle from a gained performance standpoint

Sorry for the long diatribe and I don't mean to bash on any new product that thing seems pretty cool but it doesn't change the Grendel game and considering it does not appear that the upper would be compatible with standard ar lower and Grendel doesn't stack to feed well from an ak mag what's the point

I think the idea is to have a no need to replace the bolt after so many rounds like all the AR-15 type plat forms are currently required to. Have a true 30 round magazine similar to the LWRC six 8. Some may wish to increase the limits of pressure but what little gains would not be worth it to me. There are a gazillion different 5.56 plat forms and the more the merrier with the 6.5 Grendel as far as I'm concerned. The evolution with the AR 308 plat form is a good example. I am perfectly satisfied with my Grendel rifles as is. Having a slightly heavier yet indestructible Grendel may not be for every one but I would let the market place decide that. The great thing about the Grendel is the great ballistics within the frame of the light weight AR-15 plat form. If one pushes the pressure one also pushes the recoil nullifying the best attribute of the Grendel. Just my 2 cents worth.

[mongoosesnipe]

The broken bolt issue doesn't seem to be a problem so long as you start with a good bolt and don't try to push the limit I have only heard of 2-3 bolts breaking with factory ammo so call those lemons and the vast majority of broken bolts seem to be caused by hot loads all semiautos are prone to wear and tear just using a powder operating on the slow side for a cartridge can destroy a gun in fast order by keeping the pressure high at the gas port

[mongoosesnipe]

And as far as squeezing th most from a gredel in a beefier platform that can be accomplished via an ar10 build but you can't gain much on the back end an extra hundred yards of super sonic at best the faster a bullet goes the faster it slows down, based on the size stats I don't see a significant wieght savings over just using an ar10

[LRRPF52]

If you're going to go with an AR10 receiver set, might as well go with a larger cartridge based on the .308 Winchester, but just be aware that it will be a hand-loader's affair like it is for the .260 Rem. and 6.5 Creedmoor with the longer pills.

[mongoosesnipe]

If you're going to go with an AR10 receiver set, might as well go with a larger cartridge based on the .308 Winchester, but just be aware that it will be a hand-loader's affair like it is for the .260 Rem. and 6.5 Creedmoor with the longer pills.

I completely agree, but there are those who think pushing the Grendel into 260 pressure range will make it into almost a 260 and if the can beef up an at15 into almost a at10 that will be the ticket to accomplish that, I am happy with the Grendel given performance but it think everyone who has such desires would be better served by just using an at10 in 260 and calling it a day, then the can try to push that into a magnum and see if the can find the breaking point there

[Mountain Man]

The other problem is the barrel extension, which would be the next weakest part to hold the pressure after the BCG has been beefed.

Why try to get to 60k psi in a gas gun?

A.) Because it's possible. 'Nuff said.

B.) Bill Alexander gave us a case that can handle it, even if the current preferred platform will not. (I was told the original SAAMI spec was 60k, but my QuickLoad shows 52,214- so even QL is going by application and not SAAMI specs for max pressure.)

C.) If you could shoot a 140 gr projectile at the same speed as a 123 gr (using 2375fps in Quickload), energy at the muzzle goes up 14%. (1709 ft lbs vs 1489 ft lbs) Will it matter to the Deer? Probably not. Will it matter to Elk and vermin with Kevlar hides? Perhaps.

D.) The gun really has negligible recoil to begin with. Probably less than half the kick of my .30-06 bolt gun(IMO). Another 20% increase in pressure and 14% increase in muzzle energy is not going to turn your 6.5mm into a 10 gauge mule-kicker. Heavier BCG weight might compensate for this already. A lot of people have reported success with the Tubb Carrier weight system, the extra weight is probably already incorporated into the SR25-ish bolt. Buffers and springs could be changed out, if needed for rifle longevity. I personally don't see any need to rectify the recoil, but others might, and options are available for them to do so.

E.) Grendel does the same work (until it hits pressure limits) as the 260 with a lot less powder. This is the main reason why NOT to go to a Creedmoor or a .260 rem. It is more efficient at turning small weights of powder into fast bullets. (Longer barrel life, lower recoil, and lower cost-per-shot are a result)

But, I could be wrong.

[LRRPF52]

The barrel extension is not the weak link, the barrel tenon is. The MAP for Grendel is between 50-52ksi, but I personally like to stay nearer to 50k. Yes the case can handle 60ksi, but the barrel tenon should not be subjected to that without a total redesign.

Running 60ksi in gas guns has proven to be very hard on self-loaders, as we have seen with the M4 pushing certain loads. It trashes the guns.

Running my .260 Rem AR10 at those pressures has resulted in at least 4 bent firing pin retaining pins, pierced and blown primers, violent recoil that felt like if I shot certain loads again, it would break the rear out of the extension tube...and the .260 Rem AR10 has much more room to play with pressure than the AR15 does. If you read JP's recommendations on hand loading for the AR10 in 6.5mm, he basically says you need to operate at much lower pressures if you want things to stay stable, and factory ammo is a no-go.

Looking at the overall system, how do you balance reciprocating mass, extractor life, throat life, and other critical parts with performance? Something has to give somewhere. You can get a few hundred rounds of top-end .260 Rem performance with the AR10, and then your throat is toast, bolt face looks like the surface of the moon, recoil spring is toast, gas port is eroding, and your extractor needs to be very well engineered to take a beating.

With guys who are wise beyond their years, like George Gardner, I'm seeing a trend where lower pressures are being sought for throat life in bolt guns even, and that's one of the key principles behind the 6.5 SAUM. Yeah, they could run it at nuclear magnum pressures and push a 130gr Berger at 3400fps, but why torch the throat when you can still get 3150fps at 55,000psi?

This is one of the things that has really grown on me with the 6.5 Grendel, and that is this: We are in a very comfortable place for rifle longevity, performance, low recoil, and predictable gas system behavior with factory ammunition. With my .260 Remington, none of that is true. Performance is awesome, if you like being on the ragged edge, otherwise you really need to take a step back and ask what's important to you, and how often you plan to replace premium barrels. I have gone through more replacement parts with the .260 Rem. AR10 than any other caliber, over a lower round count. Gas system signature is on the edge, and some loads have burned powder even into the gas tube, to the point where clouds of smoke literally were pouring out from the seams between the upper and lower, after an extremely violent recoil impulse. That was during load development with a certain slow burning powder from Accurate pushing 140gr A-MAX IIRC, or 142gr SMK.

The Grendel can't sustain a burn as long as a .260 Remington can, because of case capacity. That said, it still does amazingly well when you compare the 2 for supersonic reach. I actually prefer the predictable behavior of a mid-to-lower 2000fps range for close-in shots than a 2800-2900fps mv from the .260 Rem., since I've seen what that does, and it is devastating to meat if using cup and core pills, like a .270 Win. does. A .260 Rem. or .270 Win. is downright explosive at close range. They make perfect antelope rifles, since pronghorns rarely let you get in on them within 400yds, but you really need to watch the bullet construction or think about head and neck shots for close range with the super fast rifles.

When I look at what my little 16" Grendel is already doing with a 123gr using CFE, and not even pushing the pressures at 2500fps, from a practical standpoint, clarity sinks in that it's best to find that accuracy node, and get as much trigger time with it as possible at moderate pressures. I could probably run the gun ragged at 2625fps with a 123gr, but at what benefits and costs? Looking at the drop and drift, it isn't that huge of an advantage, and if I want to be competitive with a .260 Rem, I can use the 22" Grendel and push the 123gr at 2660-2700fps with CFE. The moment we start trying to push 60k in any rifle, the throat, barrel, and brass life drop radically.