You will more than likely need to use a single stack mag system if you go 243 AI. Neither one of mine feed out of the BDL or ADL bottom metal. I tried the A5 bottom metal with Accuracy International mags and worked great.
Like the idea of the 6.5-06 or 6.5-280 AI using 129 ABLRs or 140 Amaxs...
I have the 243 AI and 6x47 Lapua shooting the 105 Amax. The 243 AI going 3180 FPS with 51.5gr of Retumbo and 6x47 at 3040 FPS with 39.5gr of H4350. Both shoot under 1/2".
The 95 BTs shoot very well out of my 243 AIs with a few crows, groundhogs, and a fox. Have yet to shoot a Whitetail with one. Just killed a nice size doe with the 105 Amax in the 6x47 Lapua. Complete pass through and hit a small buck that I didn't see behind her. Went through both of his...
The 7 LRM should push 180s at 3050-3100 FPS. I plan on using 168 ABLRs and maybe the 175 in my 7 LRM build. Formed and proper headstamped brass from Gunworks runs about $1 apiece.
Wilson Combat has complete uppers or parts kits for the 458 SOCOM.
http://shopwilsoncombat.com/458-SOCOM/products/437/
http://shopwilsoncombat.com/458-SOCOM/products/436/