K51 Powered Gunner is the best loadout for him with the K121. You can use the others but the augments just don’t match up as well. Drilled is pretty much a requirement. Focus isn’t as useful as it once was though with the aimpunch/kick reduced significantly. Cool has never been useful in real practice as you’d never want to stay on the MG and fire that long since you’d get picked. But it’s still the best.
And yes, it’s worth learning simply because it’s a very different gun, it’s more powerful than the m4 and it has a bigger mag size (well, belt). I’ve got quite a few pub videos that has me playing Fragger at a high level if you’re interested in seeing how to use the gun. You do ADS it quite a bit more than other guns…especially to get that last 20-30 percent. But only for short windows of time.