The 6 Max has less a little less recoil and is a little flatter shooting than the 6ARC, you don't loose any mag capacity and it uses the same AR15 bolt , the cartridge feeds and extracts well and all at 55,000 psi which means decent barrel life all in the AR15 sized platform. IMHO, the 6 MAX is very comparable to the 6mm ARC and 22 ARC, but just checks a few more boxes and is just better ****overall**** than similar cartridges I think it would be great 5.56 NATO replacement cartridge. I bet Hornady wishes they had thought of it. I hope it catches on.
I agree with some of this (magazine capacity being the main one so far with 20rds), but I’m seeing some information that conflicts with the performance differences, specifically the "flatter and less recoil" parts. I’m looking at 6mm ARC load data and 6mm Max load data and not seeing why it would have less felt recoil or be flatter, since the charge weights are within fractions of a grain under the same bullets, with very similar muzzle velocities. The main difference there is working pressure is 3,000psi higher in the 6mm Max, but velocities might be slower even using the same charge weights.
They use the same powders and almost identical charge weights, but the pressures are higher with 6mm Max:
90gr ELD-M 6mm Max (source: Precision Ballistics Reload Data)
24” Bartlein test barrel
H4895 27.0gr 2819fps 53,536psi
CFE223 30.5gr 2944fps 53,000psi
90gr ELD-M 6mm ARC 18” Noveske (source:
Hornady’s 11th Edition, pages 239-244)
H4895 27.4gr 2700fps (24” would average 2837fps with the same load.)
CFE223 30.9gr 2800fps (24” would average 2943fps.)
Trajectory and Free Recoil energy
So once the bullets leave the muzzle, they will fly with equally-flat trajectories. For a 10lb AR-15, both will generate 4.3ft-lbs of free recoil energy. If there is a difference in recoil energy due to the 6mm Max’s higher chamber pressure, it will be academic and imperceptible to the shooter, all other components of the rifle being equal (6mm Dasher and 6mm ARC have almost the same free recoil energy). Their propellant mass, bore volume, and projectile weight will be the primary factors in free recoil energy felt by the shooter. In my experience shooting 6mm AR and 95gr or lighter 6.5 Grendel, the rifles feel like a light-loaded .223 Rem AR-15. None of them have much recoil to speak of and are very easy to self-spot with.
Hornady doesn’t list their pressure data, but they are staying under 52,000psi for the AR-15 loads, so we’re looking at either differences in propellant burn efficiency or a few thousand psi variance between test apparatuses. Since the PPC-based case uses a short, fatter propellant column, I suspect that has more to do with it because the 6mm Max guys seem to have used a reputable independent lab for their pressure testing, and the test protocol descriptions look good to me.
The real constraints
The physical dimensional constraint is the longer COL that requires different mags that will push the compatibility with the AR-15 lower receiver when you look at it from a production standpoint, especially for 25-30rd mags.
Can I make a magazine that allows 2.300” COL? Yes, of course. Can I crank out tens of thousands of them that will allow compliant factory ammunition to feed through them without binding the meplats on the front wall of the magazines for an economical price? So far, the answer to that with cartridges using shorter 2.260” COL has been a mixed bag. It also likely is factoring into why the Geissele 6mm ARC mags are so pricey. 6.8 SPC PRI mags are pricey as well. Keep in mind that with 6mm Max, we’re stretching the edge of COL just to get 100-105gr bullets in there, with powder column intrusion, whereas the 6mm ARC handles the 110gr A-Tip, 109 Berger, 108 ELD-M, 107 SMK, etc. with optimum projectile base placement relative to the SNJ (shoulder-neck junction).
If you need to use a 2.300” +/- 0.015” COL for factory ammunition, you need a wider variance length for your mag space. This means you need a larger magazine well for the AR-15, which has always been our main constraint when trying to get more performance out of the small frame receiver set. Once we depart from the existing magazine well and receiver set, we wipe out some of the important compatibility with parts supply in the market, and require re-design of the bolt carrier, its travel, the recoil system mechanical engineering for LRBHO, firing pin placement relative to hammer swing, magazine well geometry, and ground-up magazine development.
Financial constraints
You basically have to convince whoever is financing the effort that 50-100fps more muzzle velocity and 0.240” changes to component lengths is worth investing at least $1 million, likely more, for a huge gamble in potential ROI. Magazine development is very costly, as is pyramid testing.
Now if we could go back in time to 1957, I think .350 Legend or 25 Remington (.417” case head) parent cases make far more sense than the .222 Remington, and engineers within Ordnance Board proposed the 25 Remington parent with a slightly-longer receiver when they were having Stoner run around in circles with studies while trying to fast-track the M14 into production, the goal being to push any hopes of the AR-15 being adopted aside.
Right now, we do have some very significant improvements in performance out of the existing AR-15 receiver set, BCG, and Fire Control Group. The main thing I like about the 6mm Max for a military consideration is the compactness of the cartridge stack, while still bringing 8-10grs more powder under 65-75gr bullet weights when compared with the 5.56x45. That would cover down really well for both a carbine and a more nimble DM system, while still having small magazines that fit within the existing STANAG form factor for 30rd box mags. We haven’t seen a production 30rd mag yet for 6mm Max yet though, or what that would look like.
The demos where they are getting 20” 5.56 velocity out of a 10.5” 6mm Max kinda sells the cartridge for military applications, although there was pretty insane flame emanating from the suppressor in broad, cloudless daylight. It would need some flash suppressant in the propellant to help mitigate signature, like we have done over the decades with 5.56x45 and 7.62x51 in extensive continued development by ARDEC and Lake City.
