The performance for these loads is done out of a 16" barrel. FBI tests are done out of a 14.5" or 16" barrel, depending on which ones they are testing, but they have done testing on 10.5" and 8" barrels. It's very interesting to see what happens with those rounds. At CQB range, it's not too much of an issue, but once you hit 25yds, anything over 55gr starts to have issues with the really short barrels. 11.5" and 12.5" barrels obviously will do a lot better.
Every bullet has an operating range with a needed minimum velocity to expand properly. This is why the TSX loads are the best for SBRs. The 70gr TSX is designed to open up down to 1800fps, as are the other loads like 45gr and 55gr. The Black Hills 50gr TSX is a different design and is designed to operate down to 2300fps, but that's because it's a dedicated LE round with thicker petals designed to resist shearing off when penetrating through auto glass and steel. But, considering that the 50gr is an actual 5.56 load with 5.56 pressures and not .223 like everything else, they have velocities up in the 3000-3200fps range in most rifles. That gets you optimal performance out to 200yds or so, unless you are talking about stuff like a Mk18 or shorter.
Anything 55gr or less, 70gr TSX, or the 75gr BTHP or 77gr OTM are all documented to work well out of SBRs.
Last edited by Reedo302; 01-22-2013 at 10:45 PM.