I use .311" diameter cast bullets in all .30 caliber cartridges. However, some rifles may not accept a .311" bullet; .309" or .310" bullets may work better.

Try the .311" buller by loading a couple to maximum magazine length, about 2.55". If they will chamber without difficulty, they're fine. If not seat a little deeper and try again. If that doesn't work, you probably need a slightly smaller diameter bullet.

I no longer have a .30-30, but the last one I had was a '94 Winchester made in the 1980s. It worked with .311" bullets.