Return to Sender only activates 40% of the time and returns a single round to the magazine. It's decent in PvE where you can OHKO several enemies in a row and get a nice ammo boost but not so much in PvP. Mulligan virtually never activates at 5%. Perfect Balance/Explosive Rounds is nice, though. The odds of getting Perfect Balance is 33% and chances are you'll get more useful other perks for PvP, so I'd reroll it if there's nothing else you need your motes for.