I think it's pretty new that you can swing with your rifle.
I only tried it on zombies, not yet on players.

It does nothing to zombies. It doesn't hit them.
When trying to hit a zombie in the back. It just goes about its business like nothing happened. You can get as close as you want.
Needs more work. (tried it with the UMP and Mosin)

Also a smaller notice, when swinging your rifle the arms look really weird, like they're elastic and stretchy.