Long Colt - Nope! As you stated, the 44-40 bullet is a .426-.427 diameter bullet that will rattle down the bbl of a .429 44 Mag shooter, and give you lousy accuracy. Best to stick with proper diameter bullets. But, if you are shooting an older 44-40 with a large bbl diameter you might be able to shoot the same bullet from both - you would need to slug your bore to determine that or, recover one of your spent slugs from the 44-40 and mic that for a measurement. But generally, you would need tow differently sized bullets for those two calibers. HTH. Mikey.