308 Ar-10 Pistols?

I thought about making a LMT MWS 308 pistol out of a stripped MARS-H lower that I had laying around. Then I watched some youtube video where a guy tested muzzle velocities of short barrel 308 vs short barrel 7.62x39 and showed that the velocities out of the short barrel 308 was either equal to or slower than it was out of a comparable length 7.62x39. I deceided to order a PWS MK2 Mod 1 7.62x39 AR upper instead and give up on the 308 pistol idea. I'll use the MARS-H lower to build a lighter weight MWS. My current MWS is a heavy pig with DMR stock, slick upper, 24" barrel and 5-25 scope on it.