Don't bother with a rifle - use a Stinger instead. It's a shotgun, so it gives RCT instead of taking it, and has a nice focus buff. It's also armor piercing, so it tends to do more damage than a high level rifle.
Focus is good to have, but the clothes are more important. What you want is the top Pandoras Box arc 3 item - Squad Leader's Shirt. If you're having trouble hitting people, load up the IS clothes and the RCT shirt, and turn the +10 RCT ability in Rifleman on. You won't have much IS for specials, but all you really need to do is use Cripping Shot and then free fire away.