Hi
is it wrong to seize down a cast bullet to .308 for using in a 3030 gun? should it always have a bit larger diameter? why?
regards
danny

With the diameter of a cast bullet being 2/1000ths larger then the bore the bullet will seal the bore. The dreaded leading is caused by the powder gases going around the bullet and acting like a the flame of a cutting torch on the lead alloy.
Lead alloy bullets being more fragile and flexible then the jacketed cousins, having the bullet supported by the barrel aids in accuracy.
For rifles, I try to find a cast bullet that has a bore riding nose, with a rapid taper from the meplate (the flat portion of the nose) to the bore riding diameter. I've found this supports the bullets very well.
Jim

I have much experimented with .308, .309, .310, and .311 sized cast bullets in my M94 30-30 target rifle. It much prefers .310" sized cast bullets.

Bullets I use are the Lyman 311041 and the Lee C309-170-F. It prefers the Lee probably because it is single cavity vs the Lyman is two cavity.

Danny, There are two ways at determining the diameter that will give optium results in your .30-30. The first is to hold everything constant except the bullet diameter and then test at least 20 @ .308", 20 @ 309", ... .310" until you find which works best in your rifle. The other way is to slug your bore, measure the slug and then add .001". You'll still have to test 20 at say .310" and 20 at .311" to be sure. However, I'm inclined to agree with Junior 1942 about .310" being the correct diameter as my Marlin 336 does very well with it. ...Maven