As some of us have found shooting cans of soda, the container can and will rupture just as it leaves the bore, covering every one in the area with soda pop and sending pieces of can flying all over. Your paint can is even more hazardous as it is a flamable liquid uder pressure. A rupture of that can as it comes out the bore could disasterous.
So let's se if we find something better for you to shoot. First the diameter we are looking for. The rule of thumb is 1/40 of bore diameter isneed for windage. so 3.25/40= 0.08125 windage. The size of projectile you need is 0.18125x39= 3.16875.
Okay guys find him somthing safe to shoot 3.168 in diameter.
Now tell us somethning about the gun you built. How did you make the breech plug, how thick are the walls over the breech?