There are manufacturers that make winchester short mag and saum chambering s as well as many other calibers that will work on a standard dpms or Arma lite upper. Check out Accuracy Systems website. Carl can build you an upper in a variety of calibers. The SAUM case is better suited to work in standard mags. Saum cases are shorter, allowing longer bullets, and the case walls are thicker preventing case bulge/sticking in chamber. However, if you want a big caliber in an AR that is affordable and sub moa check out Noreen firearms bn36 long range assassin rifles!25-06, 270,30-06,7mm mag, 300 winmag, and 338 lapua.