If you're using a typical AR 7.62 class of magazine body, spec'd for 2.80 COL, to hold the rounds, you have to ask yourself how much you want to compromise performance?
Looking at the Hodgdon reloading page, the maximum COL for the big three .300 short magnums is like this:
.300 WSM = 2.86
.300 SAUM = 2.825
.300 RCM = 2.83
All have loadings that exceed the available length as a result of the magazine limitation, so all will be limited on some way to which bullets you can load them with. If you have to seat the bullets deeper in the case to accommodate the magazine limitations, you end up trading powder space for bullet and of equal concern is you will wind up seating some bullet ogives benneath the mouth to get them to fit. Neither is desirable. Perhaps the manufacturer of the individual uppers has a modified magazine to meet the needs of their particular caliber. Otherwise, all of them will be a compromise, with the .300 WSM representing the most compromise.
I mention this because I'm still recovering from "buyers remorse" from earlier this spring when I bought an Olympic .300 OSSM. A great concept cartridge designed to utilize the WSSM family brass, necked up to 30 caliber. All that fitting in an even more restrictive AR15 magazine. While the caliber easily delivered Hot .30-06 / Light .300 Win Mag performance, there were only a few bullets you could achieve it with. It threw 150gr flat base bullets around 3025 fps without excessive pressure signs, but as either the bullet weight went up or the BC (long/boattailed), you had to trade powder space due to the 2.26 COL magazine limitation. By the time you got up to a 180gr bullet, you were down to .308 performance for lack of powder. Still a pretty cool job for an AR15 based platform. Of greater concern was the fact that in order to use many of the bullets associated with long range shooting, their ogive was below the case mouth, so you had to be careful with any crimp you applied as well. c_bass16 has a wonderful mod to allow the bullet tips to stick out the front of the magazine and in single stack calibers, it worked great, without exposing the inside of the magazine to too much contamination. I'm not sure the .308 caliber magazines all have architecture to allow a similar mod. I know my LAR-8, which uses FAL mags, is not conducive to having a slit in the front. Other platforms may be more tolerant to that mod. I eventually sold my .300 OSSM and went with an AR 7.62 platform. Olympic recently released a proprietary lower with a slightly longer COL magazine for their WSSM/OSSM uppers, but since you're giving up the universality of the AR15 platform, you might as well just buy an AR 7.62 platform IMHO. The .300 OSSM sure turned heads at the range. Utilizing a Miculek style muzzle brake, it usually guaranteed you would have a vacant shooting position on either side of you. Brass prep was a major PITA. Sorry to digress...
Hoot