Well, the size of the drill bit is really going to depend on the size of the weapon, not having a pred with heavt bolters to hand, I can't say a specific size, but I would try to match the size of the vent ports on the side of the barrel, you can't go too far wrong then.
You might find that adding some flow aide to your paint/water mix will help with the brush strokes, as well as using thinner paint.
I have a bottle of ready made mix for my paint, it's 1 part slo-dry, 1 part flo aide and 2 parts deionised water, you might want to give it a try.
Marc