From Brotato Wiki
+60% Explosion Damage
Win a run with Artificer
Explosive Shells is an in-game item that can be bought from Shop or can be found in a Crate. Most Items increase the Stats of the player while some of them provide unique effects.
Explosive Damage stacks additively with normal % Damage, so the net gain is +45% damage with any weapon with the explosive tag, and all explosive effects.
Explosive Shells is a very powerful item if all or most of your damage is done by explosions and/or explosive weapons. However it does come with -15% Damage, so if one or more of your weapons are non-explosive consider if Explosive Shells is worth it.