Future of Intermediate Rifle Cartridges based on Grendel

[LRRPF52]

With all the Pre-SHOT 2025 announcements, this is what I’m thinking we will see moving forward.

There are no less than 4 companies making receiver sets based on the Magpul Six8 and ICAR 25 PMAGs.

LWRCi was first with the Six8 program.

New Frontier Armory makes a compatible receiver set.

Surefire is lead with their 6mm ARC ICAR program now that takes the new ICAR PMAGs.

Palmetto State Armory just announced their 6mm ARC Sabre and 338 ARC pistol.



So we have the following cartridges with SAAMI-spec ammunition/chambers, and Polymer Magazines:

22 ARC
6mm ARC
6.5 Grendel
338 ARC

Expect to see more 22 ARC and 6.5 Grendel factory rifle/carbine options using the new receiver sets.

Expect to see Grendel-based wildcats looking back at different bore diameters.

These new firearms have different/stronger bolt and extension designs.

The 338 ARC will take away a lot of the appeal of the 300 Blackout and 8.6 BLK.

[A5Blaster Number 2]

Not to long down this road we will see steel body mags to fit this sized magwell and 2.5 inch coal length cartridges with PSA's newer bolt and barrel extension.

You can almost guarantee it.

[grayfox]

So with the extra available OAL, and stronger bolt/extension, does this also mean that the distinction between AR and bolt action pressures/MV's will be eliminated?

[biodsl]

So with the extra available OAL, and stronger bolt/extension, does this also mean that the distinction between AR and bolt action pressures/MV's will be eliminated?

Inquiring minds want to know! Some yahoo dist me when I asked this question in the comments on the PSA Sabre introduction video. Any chance the new bolt can handle 60K psi?

[A5Blaster Number 2]

So with the extra available OAL, and stronger bolt/extension, does this also mean that the distinction between AR and bolt action pressures/MV's will be eliminated?

Well the polymer mags don't offer much more oal, compared to metal grendel/arc mags available for standard AR-15 magwells. 2.315 to 2.320ish max length in the 10 round mags.

But a steel body mag for the six8 sized magwell would allow a whole hell of alot more. It's been a bit since I took the measurements. But I believe I came up with a steel mag that would have around 2.600 cartridge length space.

[LRRPF52]

I just measured my New Frontier Armory 6.8 lower receiver magazine well raw internal dimensions.

The maximum raw length without any magazine is 2.475”.

A5Blaster: Do you have the LWRCi or NFA? I thought you had LWRCi. Would be interesting to compare but they should be about the same.

Once you add magazine wall thickness on front and back plus stand-off, you won’t be much over 2.300”.

Raw internal length in a Six8 PMAG is 2.327” without the follower, so I could get down deep into it with the calipers.

You need a different action or the mag well enlarged rearward with new mags to get 2.500” COL allowable, like what FN did with the LICC.

If you look at all the 6mms with .473” case heads and compare them to 6mm ARC, the juice isn’t worth the squeeze for the 6mm BR, BRA, Dasher, GT, or even 6 CM.

6.5 Grendel with 100gr already does 2800-2900fps from a 24” barrel without chasing pressure.

Those larger cartridges need 62,000psi and 26-28” barrels to push a 6mm 105gr that fast. 6CM will do it from a 24” barrel at 62ksi.

The little Grendel case is extremely efficient even at 12,000psi lower chamber pressure.

[A5Blaster Number 2]

I just measured my New Frontier Armory 6.8 lower receiver magazine well raw internal dimensions.

The maximum raw length without any magazine is 2.475”.

A5Blaster: Do you have the LWRCi or NFA? I thought you had LWRCi. Would be interesting to compare but they should be about the same.

Once you add magazine wall thickness on front and back plus stand-off, you won’t be much over 2.300”.

Raw internal length in a Six8 PMAG is 2.327” without the follower, so I could get down deep into it with the calipers.

You need a different action or the mag well enlarged rearward with new mags to get 2.500” COL allowable, like what FN did with the LICC.

If you look at all the 6mms with .473” case heads and compare them to 6mm ARC, the juice isn’t worth the squeeze for the 6mm BR, BRA, Dasher, GT, or even 6 CM.

6.5 Grendel with 100gr already does 2800-2900fps from a 24” barrel without chasing pressure.

Those larger cartridges need 62,000psi and 26-28” barrels to push a 6mm 105gr that fast. 6CM will do it from a 24” barrel at 62ksi.

The little Grendel case is extremely efficient even at 12,000psi lower chamber pressure.

I have the LWRCI receivers and original Magpul 6.8 pmags

My memory could be wrong on the measurements, it was a few years ago. Need to sit down and take them again to be sure.

[grayfox]

...
If you look at all the 6mms with .473” case heads and compare them to 6mm ARC, the juice isn’t worth the squeeze for the 6mm BR, BRA, Dasher, GT, or even 6 CM.
6.5 Grendel with 100gr already does 2800-2900fps from a 24” barrel without chasing pressure.
Those larger cartridges need 62,000psi and 26-28” barrels to push a 6mm 105gr that fast. 6CM will do it from a 24” barrel at 62ksi.

On the smaller ones like 6BR, BRA, dasher, I don't have those so no comment there.
But I do note that in my 6 CM Bergara, I've got chrono with 103 Eldx and Staball, at 2925 fps (good node) and only ~55ksi at 40.8 grs. When I "cut" to 24" from the 26 in grt, it's still 55 ksi (peak is unchanged) and 2876 fps. Hornady book max is 43.3 grs and 3096 MV. So the point may well be a good one, but there's still some room , IMO, for the 6 Creed. Another pside for 6CM is this is with pretty stable powders, vice needing Lever in the 6Arc for example (I'm not against the 6Arc and lever, just saying). Now the downside of the 6 CM is that it is a barrel burner, but that might be at the screaming MV's of a long range/PRS/ benchrest shooter. Even in mine I have detected some throat movement however, so I'm watching it.
For my "fave" 107 SMK, 2960 fps with Staball and 2940 w/ H4350, and both ~2 grs below max ( max MV's 3096-3100, 42.1/43.3 grs). This is the 26" barrel chrono's. And ~0.5-0.75" at 100 yds.

[m796rider]

I would love to try 6GT in my POF Rogue, but I'm super intrigued with the possibilities of 6ARC in PSA's updated AR platform. I just started shooting 6MAX, but oh boy how things have changed in 2025...

[grayfox]

OK, still waiting on whether this new bolt can take us up to 60ksi in the AR.

Here's another.
2, With some extra length in the mag, does this allow for a "6.5GrendelXL", say 0.2-0.3 inch case body extension (I'm not greedy-)? They're already making a non-mil spec lower, right, why not add this into the mix.Push the forward end of the lower forward 0.2-0.3" if need be.

[LRRPF52]

If you had an AerMet bolt and extension, you could run 62ksi.

When Remington did the R-15 in 30 RAR, it had an AR-10 diameter bolt and extension, but AR-15 bolt lug lengths. They limited it to 55kis due to lug lengths and maybe hoop.

If we look at 6.5 Grendel right now from a 24” and compare it to .260 Rem 24” with the same bullets, you see the ballpark you’re looking at playing with in-between.

.260 Rem 24” 60ksi

100gr 3100-3200fps
123gr 2850-2950fps
130gr 2750-2850fps

6.5 Grendel 24” 50ksi

100gr 2800-2900fps
123gr 2550-2650fps
130gr 2420-2550fps

We’re basically 300fps avg between the 2 from the same barrel lengths, same exact bullets.

How much effort on the back-end is it worth to close that gap and how much of the gap will we reasonably fill-in?

What I saw in the Howa bolt gun when I pushed it was maybe 100fps increase with the 129gr SST, and that involved sticky bolt lift.

[biodsl]

If you had an AerMet bolt and extension, you could run 62ksi.

So, who will do this? Surefire?

What's going on that initiated the renewed interest in the LWRCi, Six8 form factor? Is all of this happening because of the Grendel "weak bolt" legacy? Curious if there's a military interest in this.

[grayfox]

+1, it also doesn't make sense to me that max ksi would be unchanged for this new "stronger" bolt mentioned above.
Why do that if it's not needed?

[LRRPF52]

So, who will do this? Surefire?

What's going on that initiated the renewed interest in the LWRCi, Six8 form factor? Is all of this happening because of the Grendel "weak bolt" legacy? Curious if there's a military interest in this.

Sons of Liberty Gun Works made an AerMet Grendel/ARC bolt due to requests from people shooting a lot of gas gun PRS-type matches.

You really have to push the loads and shoot high volume to get a quality bolt to fail.

I’ve been shooting Grendel regularly with multi-day courses since 2009 and still haven’t been able to break any of the bolts I’ve used, and I’ve been shooting 12” CLGS suppressed the most over the past 7-8 years, along with 18” MLGS Grendels. Those combinations are the hardest on bolts.

The real focus on the Surefire ICAR seems to be the magazine, where people kept asking for PMAGs.

Military end-users don’t necessarily like the long Geissele 30rd 6mm ARC steel mags because they don’t fit in existing pouches, and the Geissele mags cost $128 each.

So for a civilian competitor who wants to have 4-7 mags is looking at $512 to $896 just for magazines. That’s out of reach for the layman and even lifts the brow of the high-end buyer.

[Fess]

Inquiring minds want to know! Some yahoo dist me when I asked this question in the comments on the PSA Sabre introduction video. Any chance the new bolt can handle 60K psi?

It would not be hard to do. Running bolt thrust numbers, even a 65Kpsi cartridge would have less bolt thrust than the 30 Rem AR, which was in a modified AR15 platform. That cartridge matched the 308's bolt thrust, FWIW. Both the GMMG Mutant and Colt CM901 (the 7.62x51 AR15 variant sold to Yemen) have run AR10-diameter bolts and corresponding barrel extensions, so it has been done before on a commercial level. To equal the bolt thrust of a normal SAAMI 308, a Grendel would be running at a little over 70Kpsi.
For some reason, Geissele decided to use a new alloy and forge the bolts in its 6mm ARC GFR rifles, but they have not stated what ultimate chamber pressure it can tolerate. Then again, those rifles start at $3,200, so they were not necessarily looking at the least expensive option.