If I wanted to use the same mags, lower and bolt with a second upper with a different barrel, what are the most viable options? Of course there's 300 BLK, which seems to have leapfrogged all others. But what stacks up well for supersonic loads for distance shooting? There's so dang much, 25-45 sharps, 6x45, 277 wolverine, 243 mongoose, 20 practical, etc. I'm not even sure why so many exist, there's so much overlap in bullet diameters and weights - plus they all have the same parent and work on the same platform.