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.

Thanks, what's a good perk set up for PvP? Also how useful is explosive rounds in PvE/PvP?

Explosive Rounds are moderately useful in both. In PvE you can deal a bit of extra damage if mobs are grouped tightly together and, well, they're fun to use. In PvP they don't add any damage and on some guns, lower the overall precision damage. Their main effect is that they shake the camera of your target and disorient them as well as (at least it seems) deal more flinch. Their effectiveness in this regard was lowered in the latest patch. As for PvP, good sets are listed in the guide. I'd go for: Perfect Balance/Luck in the Chamber if you don't like the recoil. Or, Flared Magwell/Luck in the Chamber Field Scout/Crowd Control Flared Magwell/Reactive Reload if you have no problems with the recoil.