Pistol only relic in custom matches (extinction)

I've found that, in custom matches, if only 2 or 3 people are playing and everyone is wearing the pistol only relic, you still get weapon challenges such as "Kill 20 enemies with shotguns".

I don't know if it happens if 4 people are playing but I know it doesn't in individual play.

By the way, I play on PS3. Thanks