Ozutsu are heavy muskets, verging on being hand-held cannons, that pack a serious punch. Warriors armed with them commonly tied a strap of cloth around the barrel and their wrist to help control the recoil.
They give you a whopping 5 dice on the Damage roll but with less range than a normal musket - 16". In addition they can't move during the Shooting action so you'll need to think ahead exactly where to move them. You can find full details in the Sengoku expansion book.
Upgrade them further with either of the two Trait cards included:
Smoke Clouds represents a particularly sooty batch of gunpowder, providing some handy cover for the rest of the turn after shooting.
Too Much Gunpowder boosts your Damage rolls with an extra dice, at the risk of blowing up and taking out the shooter!
Note that both these Traits can be used with muskets of any type.