Re: uncommon chamberings for AR10 platform
The list is damned near as long as the AR-15 caliber variant list. Plenty of commercially available cartridges, along with a host of wildcats, for the large-frame AR platform. Here's just a very basic list of some alternative calibers:
.243 Winchester
.22-250
.257 Roberts
.260 Remington
6mm Creedmoor
6.5 Creedmoor
6.5 WSM
.270 WSM
7mm WSM
7mm-08 Remington
.284 Winchester
.300 WSM
.300 SAUM
.300 Savage
.325 WSM
.338-7mm Benchrest
.338 Whisper
.338 Federal
.338 RCM
.35 Remington
.358 Winchester
.358 HDH (WSSM)
.450 Marlin
.500 Phantom (this one may have died on the vine)
There are more I'm sure (not counting numerous wildcats), but that's a short list. Personally, were I going to go with a one-off or otherwise not quite "mainstream" chambering for a large-frame AR right now, based on stretching the legs of the platform perhaps as far as they can go, the 6mm Creedmoor based on my research, what I hear from the fine folks at GAP and other members here, would likely be my choice for a complete across the course, long-range and even light hunting large-frame AR. As for "components" on the 6mm Creedmoor, I haven't spec'd one yet, but once things settle down a little with all the AR craziness, etc., I'll probably have GAP build me a complete upper for my P-308 in that chambering.