The rifle has three things going for it:
- A slow effect with a “very” generous area. It’s better than the shortbow for this. Slowing is one of the most powerful effects in the game, almost as good as stun, as it keeps enemies from being able to mob you all at once.
- Rifle ignores block, which makes it excellent at killing splitters.
- Assassinate is OK at killing big nasty enemies.
If you try to set up a hero with a rifle as their only weapon using the “specialist” perk, you’re going to be disappointed. Maybe if you stacked the hell out of additional action points you might be able to get it to work, but it would be a hard build to pull off. The rifle works best as a secondary weapon that you use for its area-effect slow and to hammer enemies that have a high block value.
There is a perk that gives you +30% dmg if you use only 1 weapon set (that is must take I think). So there is no point in 2 weapon sets and the ap costs for rifle too high. So the only use of it I see is killing fat isolated enemies with his 4th skill, but good aoe later in the game so much better.