Mountain really covered the bases here.. I've had mine, a Rock River Arms upper for about 3-4 years now.... love it! I did lots of research before purchasing, checked out the 450bushmaster, the .50AA and maybe another in that class, after weeks of research, I came away with the 458 socom, hands down. Some of the reasons for going with the RRA were related to dependability and performance. They had a really good bolt and the carbine gas system was the length of choice that worked best. I spent lots of time reading on the 458 Socom board I found, tons of great experience there to draw from also the history of the 458 socom and where and how it came to be. I actually used it for bear hunting this past season, but didn't get an opportunity to use it.. felt really confident carrying it ..
I to used 20 and 30 rd Colt preban mags, modifying the front of the mag to get good upward motion of the bullet into the chamber.. works great. I use the same mags for my 300 aac black out.. another upper I absolutely have fallen in love with. I used that for coyote hunting..
hehehe, didn't get a shot at any of them either .. The one point I'll make, you don't have to spend 1500 for a good upper, the RRA has been fabulous and flawless.. think I paid like $750 with the bolt and charging handle new.