The drill bit (in the pic) looks too thick for your main gun barrels.
Assuming you are using hand drilling (ie with a pin-vise). Before you drill it, use a needle (or anything pointy like a pin) to poke the centre point of the barrel. Sand it flat if there's a seam, which they typically do as part of molding a cylinder to avoid the seam interfering you finding the true centre, if you need additional help, mark the centre with a pen before you poke it.
That little hole from the needle will act as your guide when you drill with the drill bit.