BANG!.cz - Fórum

Sniper - During your turn, you may discard 2 BANG! cards together and target a player: this counts as a BANG! but it may only be cancelled by 2 Missed! cards.

Does that actually mean "Missed card" or "Missed-effect card"?

It means "Missed-effect card", you can use the Barrel. Like with Russian Roulette according to the FAQ.

The misunderstanding is probably due to different release date of the expansions: when AFOC has come out maybe there was no need to explicitly differentiate 'card' and 'effect' meanings