lead bullets are not for use in the ruger auto 44's.
from rugers instruction manuel:
Do not use lead bullets -- they will clog the gas port in the barrel and prevent proper action cycling. Aluminum-cased ammunition should not be used, as it may stick in the chamber when fired, resulting in extraction and functioning problems. .44 Magnum cartridges with bullet weights below 215 grains are generally not compatible with the Ruger Deerfield Carbine, and those with bullet weights above 255 grains generally exceed the maximum overall length of 1.610 allowed by U.S. Industry Standards and may not fit into the magazine.
i would say either a 240 grain soft point, or a 300 grain bullet.