If my memory serves right, Impel Down had seastone bullets, and even tried to use them against a fruit user. They also had a seastone net.
The problem is, you still have to hit, and hit well. There are characters who can tank bullets even if their powers are turned off. Plus, seastone is expensive, and it seems they need most to go in and out of Grandline. I can understand them not being happy to spend such precious material on munitions.