Thanks goes out to the members of AR15.com. This forum has provided a great deal of information used in the development of a new AR-15 wildcat, the 400AR.

Using 10mm/.40 caliber bullets, the 400AR is the highest caliber that double-stacks in AR-15 magazines. Taking advantage of big bore energy, current standard loads produce 2,374 ft-lbs @ 2,312 FPS with a 200 grain rifle bullet. Experimental loads with 135 grain pistol bullets have produced 2,774 ft-lbs @ 3,042 FPS while cycling the action at safe pressures. Its head size allows a pressure rating of 50,000 psi using an off-the-shelf 7.62x39 bolt. The only custom parts required for the 400AR are the barrel and a modified magazine.

Details: https://400ar.wordpress.com/

Questions, thoughts, opinions or ideas?

Looks like fun.  I like the idea of a big bore cartridge double stacking in the magazine.

Any other magazines being considered?

Looks interesting. Where do you get the 7.35x51 Canaco cases from? I have been looking for some for my rifle.

Pmags were first used by removing the shoulder ridges. They functioned, but were too tight for the stack to nest well. So attention was focused on using modified 6.5 Grendel mags. Removing the shoulder ridges and cutting back the lips from a 5-shot Grendel magazine was a slam-dunk, works like a charm. But after removal of shoulder ridges on a hicap Grendel mag, a broader spring was necessary and was fashioned from one out of a VZ58 mag. Works ok, but isn't very pretty.
Know of a good supplier of custom magazines?

Got the 7.35x51 Carcano cases from Buffalo Arms, but I'm not sure they still carry them. MidwayUSA has them now.

In the right mag, does this round double stack?

It has a x39 head size, taper like a 5.56. It double stacks.

Looks cool. What barrel lengths are you using?

This.  Great idea, but with only 5 round mags available, having a cartridge that double stacks is pretty pointless.  You would be better off with a 458 Socom or 50 Beowulf with 7 rounds in a standard 20 round GI mag.  Have you considered necking down to say, .375?  It may allow you to use standard 6.8 or 6.5 mags and still pack big bore thump.

TROMIX is already doing a 10mm SOCOM although at this time there are no plans for bringing it to production.  There is really only one bullet that is suitable for big game, the Hornady 200 gr. muzzle loading SST sans sabot.  I am getting close to 2200 fps with that bullet (16 inch barrel) in the 10mm SOCOM with sub MOA accuracy for 5 shots at 100 yards and it holds up perfectly.  The Hornady 200 gr. XTP fragments badly at 10mm S velocities and lighter bullets fragment even more so.   The 135 gr. Sierra JHP (2400-2500 fps) did not even penetrate one one-gallon jug of water completely, but by God it blew the living hell out of it!
I have talked with bullet makers and LEE and they agree the .411 Hornady 300 gr. bullet can be sized down to .400 and work OK, we think, but I see no reason to test it with the 200 gr. SST working so well.  Shot into water at 50 feet the bullet blew apart 4 gallon jugs of water, pencil holing through the 5th and stopping in the sixth, with retained weight of about 150 gr.  It is reported in more detail on the 458 SOCOM forums in this link.

Just like the other SOCOM based rounds, .475 TREMOR, .458 S, .375S, it operates at a max pressure of 35-37K psi and uses unmodified AR 5.56 mags.  Case life is outstanding just like all the rest and bolt life is not shortened due to higher pressures and backthrust.

If I were building it, I would have started with the 7.62X39 case necked up to .410.

That way you could use .41 caliber handgun bullets, or .411 rifle bullets.

It would be a sort of AR15-400 Whelen.

G.

This would have been a better option. Still interesting wildcat round. I reload 40 S&W. was considering a carbine. I like to keep my reloading simple by staying with certain calibers. Will keep an eye on this thread. May have to consider this down the road if more support follows.

From some of the comments I'm not sure it's understood that what we are looking at is a wildcat, a one-of-a-kind prototype. If you want one, you'll have to build it yourself. If you wanted a magazine, you'd have to modify it. If you want ammo, you'll have to neck out and resize brass to load it. That's part of the fun of wildcatting. The rest of it is in the possibilities.

Ditto with the TROMIX 10mm SOCOM.  It would have to have a whole lot of interest before it would even be considered for production, at least 100 people committing to one.  With what I consider only 1 bullet choice available for big game, unless one wants to invest in bullet forming, that is a hard row to hoe right there.  In the 10mm S case, the intermediate form die for the .375 SOCOM is all that's needed to form the 10mm SOCOM, so for us that already are loading the .375S, it's an easy conversion.  Barrel, .458S bolt, and done.  A quick pass through the .375 intermediate die and you have a fully formed 10mm SOCOM case.  But with only one suitable big game bullet, that really limits its usefulness.

However, as a CQB round where very limited penetration might be wanted, then the 135-200 gr. pistol bullets do show potential.  The speed and accuracy is certainly there and the results of a pistol bullet impacting a target at twice the designed speed is truly spectacular.  

Is there a burning need for such rounds as the 10mmS and .400 AR?  Not really, but that is not what wildcatting is about.  Sometimes wildcats catch on big time, but that is certainly not the norm. Most cats are total failures even from the standpoint of the developer.  My .338 GRRR is a total flop, but I'm still glad I did it and I learned a lot.  Wildcatting is about doing things not done before and once in a while, a true jem is developed, like the .458 SOCOM, which then gives rise to even more wildcats, for better or worse.

Besides, wildcatting is just plain fun.

One place this cartridge might have is in the states where hunting with "pistol" caliber carbines is newly legal, like Indiana and Michigan.  

Michigan requires a straight sided cartridge, .35 caliber or larger, with a case length 1.16" to 1.80" in length.  

This would fit the bill, and with the polymer tipped bullets, would be a flat shooter.  probably an easy 225-250 yd. rifle.

With a complete upper, this could be a competitor to the 450 Bushmaster or the 50 Beowulf on the AR type rifles.

...

This.

That was considered, but the 7.62x39 case is too short and too tapered. Using .41 caliber was also explored, but would have made a cartridge with too little taper. Using the Carcano case with a .40 caliber bullet, the 400AR is just right.  

Is this ready for open  market yet, looks like a great idea

 I believe I get mine from Gaff & Son.

Anyone making the chamber reamer for the 400AR? Sounds like a good round.