I would like for all of you to subscribe to John Farnhams dt quips, seems the heavier cartridges in the .556 lowers are breaking things. Opening the bolt face is weakening things. The calibers are sound, just overstress the smaller caliber equipment. Going with the larger AR platform, lots of stuff is cool, I too am a big fan of the 6.5, it has everything going for it, SD, BC and inherent accuracy.
Years ago, Springfield made M1'as in .308, 7mm/08, and .243 I have 2 in .308, but would love one in 7mm/08, 1 of my favorite calibers for a long time. Today if the .260 were available it would be tempting.
DPMS uses FN mags, = cheap.