I don't know about the 7400, but the stuff you generally see about semi-auto 308 appropriate powders has to do with loading for the M14/M1A type rifle.
It's not just about being fast enough. It's about being in the proper range to provide the correct gas port pressure to properly cycle the action. The 4895 type powders are pretty much in the middle of this range.
With a 308 it's pretty hard to go wrong with a 4895, either H or IMR. Ramshot TAC is a modern ball powder in this burn rate class, but I haven't tried it yet.
Be sure to be careful with loading data that was developed in commercial cases that have greater capacity than military ones.
