The baseball grenade is an explosive weapon in Fallout 4.
With the surplus of baseballs around Boston, tinkerers converted these common playthings into low-tech explosives. They may not pack the punch of a military grade fragmentation grenade, but they are close enough for any wastelander.
Baseball grenades are weaker and heavier homemade imitations of the standard military fragmentation grenade. The cork core of the baseball has been hollowed out, leaving only the leather covering, then packed with a volatile mixture of fertilizer and oil, along with scrap steel to form the fragments upon detonation.
By default, baseball grenades can be created at a chemistry station. With the Contraptions Workshop add-on, they can be manufactured using an explosives mill.