Start about .002" to .003" over bore/slugged diameter. If a revolver, the same size or slightly (.001") smaller then cylinder throats; a push-through fit. In my revolver the cylinder throats measure about .431", and the bore is .429", so I use .431" to max. .432" bullets. Very little to no leading.
The rule of thumb is .001 over actual bore diameter (not nominal bore diameter...in other words, a .308 may not be .308 inches in diameter. If it's .309, size your bullets to .310, etc.). This is a starting point, not the final answer. You'll have to experiment a little to get the optimum load for accuracy, no leading, etc. If you're getting leading you could tweak the velocity with the same diameter, or tweak the diameter and leave the velocity the same -- there are a TON of variables. Btw, I suggest using gas checks in rifles.
What the other's say is what I have found also in my newly discoverd casting hobby.
.001-2" over groove dia. However, first things first,,,,see what dia. your mold throws with your alloy first. Load one dummy up as cast, and see if it chambers ok. If so, sizing down really isn't necessary. Then too, what does your sizing die actually size to. Some are too small and you may need to open it up some. Ideally you want everything just a tad snug. As in slight burnishing of the bullet in the throat, with the loaded round, seated so the bullet is bumping the lead enough to see a slight engraving. This reduces skidding, if you are casting just a tad on the soft side for your velocity.
You want reliable feeding with just a lil effort to close the action. If your throat is a tad big, the barrel will swage the bullet down no problem.
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