I looked at both the 6.8 and 351SLR cartridges. I was thinking if it was possible to swage a belt into the 6.8 and blow it straight out. It would be in the 35 cal. family. OTOH, do you know if anyone has just rebarreled any AR for the 351SLR ?? I'm a left handed person and I always wanted a Win. Model 1907, in 351. But I didn't like all the flash bang right in front of my nose. A BHW left hand upper seems to be a viable answer to my concerns, and the 351SLR may be your answer, too.
My dad saw blue prints from Marsh Carbine Williams in the Frankford Arsenal, in WWII, which were for gas operated Win. 351's, and 401's.
He shipped overseas, and never heard if they made up any. If they did, these would have been our first assault rifles.
In reality with its long bullet protrusion from the case, the 351 is about on a par with a 357 Maximum. The similar 401 is more like a .41 super mag. Case capacity is the same as the 41 S&W Magnum, but the O.A.L. is close to two inches, and the bullet doesn't go too deep into the case. These chambers were designed from the beginning for this. So standard 351SLR and 401SLR reamers will have all of this engineered into them. With a fresh barrel, these two make, very, very, accurate rifles.
There were once 200 grain loadings for the 351SLR. I think the big brother, 401, only had a 240 heavy bullet, but Lyman made a .410426 mold which I once used in a couple of 41 Mags. Cast really soft, it got up to 250 grains, without the gas check.
I always wondered about a 501SLR, which would have gone from say 340 up to 450 grains of lead. If one can make 351 SLR cases out of 357 S&W Mags, then maybe you can make semi rimmed 501SLR wildcats out of 50 Alaskan brass from Starline. Single stacked, the AR should give one an extra quarter inch protrusion over the other SLR's factory O.A.L.'s of less than 2 inches. The 500 Wyo. Express is the closest factory ammo available today. F.A. tells you not to hot load it in their light revolvers, but in an AR frame, these heavyweight loads may be just what the doctor ordered. Crimping with a Lee FCD, farther down the bullet shank, to give a longer O.A.L. than the F.A.'s 1.6" , could be a quick way to experiment. F.A. feels that the increased recoil of these fifties, make a good roll crimp necessary. That rules out cartridges head spacing on their case mouths. The F.A. 500 W.E. is a Jeffery case head design. I think I would go with shortened Rem Ultra Mags or S.A.U.M.'s., for their better extraction groove, and better still, swage a belt more beefier than the skimpy one on the 500 W.E.'s.
So from the 17-223 to the 458 Socom, we are finally understanding what the parameters are for this platform. Now things start to get interesting.