On a range day larf, while shooting my semi-dedicated .22 upper (it has a CMMG conversion bolt installed full-time but I can drop my 15 carrier in if need be) a friend and I had a discussion about uppers, caliber versatility, ammunition commonality and the possibility of building up a Marlin 1894 as a kind of scout rifle, throwing a low-powered optic on the thing and firing the same ammunition I shoot through my preferred sidearm (SW 586 .357m).

Then the gears began to mesh and we had a eureeka moment. Why not just drop on a .357 upper onto my existing lower?

So. Has somebody done such a thing? I did a little googling about and discovered a thread over on THR where they briefly mentioned ".357 Magnum Rimless" as an AR upper variant, but didn't really go into any detail on it. I don't really know what a Magnum Rimless is, but I'm just going to take an educated college-boy guess and assume that it's not the same cartridge as I fire through my revolver.

I was thinking that it wouldn't be entirely too difficult to build up a .357m/.38spl upper. It'd simply require a modified bolt face (wider for the rimmed case, with a deeper extractor), a 9mm barrel chambered for the magnum cartridge and a half-staggered magazine like those Black Dog makes for the .22. Naturally if they were in the 30-rounder form factor, they'd be limited to a capacity of 10-13 rounds.

So, has anybody already done all the hard work on this? I didn't find too much via google and figured I'd pick y'all's brains.

The problem is that the .357 Magnum is a rimmed cartridge and typically semi-auto ammo is "rimless" ammo.  So there are bolt issues and even beyond that magazine issues with a rimmed cartridge.

I'd love to see one since I have a considerable supply of .357 Magnum and only a couple of Winchester lever-actions in .357 mag!

Yeah well, I've got like three autos that take rimmed cartridges (Ruger Mk.2, .22 AR upper and a 10/22). I don't see why it would be so challenging to scale their setups up.

Actually, come to think of it, doing a bolt conversion like the CMMG (Ciener) kit might be the best way to go on this. Magazines naturally will end up being a pain in the ass to figure out.

It can't be a staggered column mag I don't think. I remember hearing that it's very hard to get rimmed rounds to feed reliably in a double stack. If you have some kind of luck, tell me cause I'd be interested.

Also with some of the hot .357 Mag loads... would they be too much for a blow back like the 9mm uppers, or would it be necessary that it be a gas operated upper?

T ArmaNotSoLite, I was mistaken, it wasn't the Black Dog magazines, but the S&W .22 AR that has the half-staggered magazines. Observe:

Now, puzzling this out a bit more, I'm reminded of an Ishapore 2A I owned years ago and the differences between the bolt face on the .308 2A vs. the bolt face on the .303 No.1mk.3. The extractor claw on the rimmed .303 was wider, flatter and deeper than that on the .308, other than that, it was basically shaped the same.

Thinking a bit more about it, I'm wondering about the possibility of using a .40 or .45 bolt face rather than a 9x19 as I'd hate to take off too much of the face with the locking lugs, doing so may cause safety issues.

T DasRonin, I think you're right, it may have to end up being gas operated. Scaling up the .22 bolt may not be able to handle the pressures involved with a .357m cartridge.

If someone was so inclined, it would be possible...

There is that .410 upper out, and it has 5 and 15 round magazines, both are single stack. If you started with that bolt, might be easier to put in a .45 Colt/.454 Casull/.460 Magnum.

how bout something like a 357sig long (sounds like fun to me )

could you take a .450 and neck it down and with a 146 grain pistol bullet your OAL could fit nice

357RMW
357 Sig dies & reamer using 10mm brass. 115gHP at 1800fps & the 147gHP at 1500fps with a 10.5" tube.

in was more thinking 9x40mm  I know what a 9x25 is but never hear it listed in an AR thanks
off the .308 case maybe
147gr JHP at maybe 2400fps at those speeds the round would act like a 115gr and frag

not sure if there would be feeding issues tho with that blunt nose (or bullet setback)

I don't have any .357squig. Although making one would be as trivial as taking one of the .40 bolts and chambering a 9x19 barrel to the squig spec.
This project isn't about doing things the easy way, it's about doing something unexpected.

I was thinking there'd need to be a feed ramp integrated with the magazine. I was thinking about having a little ramp at the front of the magazine with a spring-loaded detent at the top. The cartridge rides up the ramp, but as the cartridge enters the chamber and the bolt rides into the locking lugs, the little ramp tips out of the way.

Either that or you know, build a ramp into the front of the lower. But psssh that's all like, easy and reliable and stuff.