You also need the cylinder throat diameter to be equal to or greater than the bore diameter. If this is not true it does not matter what you choose for a bullet diameter, it will still give you leading.
This is 100% true and why it is sometimes necessary to have the cylinder throats enlarged to allow shooting of lead bullets without leading.
A few comments to add to what Bomberman said:
- make sure you use PURE lead when you slug the bore. I use muzzleloader balls since they are pure lead and you can obtain them in the correct diameters
- oil the bore before you slug it, it will reduce the effort to drive the slug through
- while I have successfully used an oak dowel to drive the slug through, I switched to a brass rod. 3/8" works fine in 44 Mag. I have just read too many horror stores about a wood dowel breaking an wedging into the barrel. Removal is very difficult. And the brass rod is necessary if you ever have a jacketed squib bullet you need to remove.
- I have never been able to accurately measure the groove diameter on a 6 land barrel using a caliper. I use a micrometer that reads to 0.0001". But with the 5 lands and groove of the S&W, you just can't an accurate measurement. The proper fixture micrometer runs over $200 new. One of the cast bullet producers offers to measure your groove diameter on the 5 land slug using the proper micrometer. Can't recall his name right now.
- when slugging the barrel notice if there is a constriction where the barrel passes through the frame. If there is, you may have leading issues due to the reduction in barrel diameter at this point.
- if you slug the bore and it passes through the cylinder throats as mentioned, then you can check your cast bullets in the throats to check. Pin gages are an indirect way to check the throat diameters, but the minor and major diameters of the throat will vary somewhat.
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