ORIGINAL: dmacl
You'll want to slug your barrel and make sure you order properly sized conicals if you are purchasing single diameter conicals from a custom caster.
You lost me here. Slug my barrel?
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You need to know yourbarrel's land diameter. Barrels aren't always consistent for a make or model. Slugging will help you to determine your bore's precisediameter. Most folks are having them sized that size or just slightly over (one pass instead of 2 through the sizer). Cast bullets don't always come out of the mold the same. Depending on mold temp and so on. So they are sized to the proper final diameter. You need know what that is. If you don't, the bullets you buy may go down too hard and that can also affect accuracy.
I'd probrably mess up telling you how to slug your barrel. So someone who has done it should describe the process.