For jacketed bullets use the measured slug dia..
So, a .308" slug measurement would need .308" jacketed bullets.
For lead rifle bullets go actual slug dia, or slug dia. + .001"
For lead bullets in revolvers, measure cylinder throat dia, and if they are bigger then the slug measurement, ignore the actual barrel dia.
The reason for this is many older revolvers had very over-size chamber throats. You want the bullet to fit the throat when it comes out of the case, and then it will slug down to fit the bore if necessary.
say I slug my barrel and the bore is .xxx and the groove depth is .xxx+.010
When you measure the lead slug O.D., you are measuring the actual groove diameter of the barrel. The actual bore dia is at the bottom of the grooves on the slug, and are very difficult to measure.
Just measure the outside of the slug on the high points of opposing rifling marks, and do not add .010" to it.
You can never assume all rifling grooves are going to be .005" deep because very few of them are. For instance, most conventional rifled .308 barrels have a .300" hole drilled & reamed through it, and the rifling grooves are then cut .004" deep.
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