The Multishot enchantment is a rare and it adds extra arrows to the damage inflicted by an arrow hit. On multiplayer, any of the players can get this effect. The best way to think about the enchantment is that every consecutive shot will do 15% more damage than normal as long as there are arrows left in your inventory for each successive shot.
In other words, single-shot weapons deal 100% base weapon damage, while firearms with multishot have their base default reduced to 50%. If you have a 1-enchanted firearm that does 50% increased base weapon damage normally, then on its first shot in combat it deals 625%; but on its second and third shots it would only deal 5625%, since those arrisese from the initial shot’s base damage.