Sverre,
If I understand your question correctly, you are asking if your Winchester lever action rifle (model 1894?) will handle hardcast 0.3" diameter flat tipped bullets. That probably means your rifle or carbine is chambered for .45 Colt or .44 Magnum.
YES, the Winchester 1894 will safely handle the full range of factory loaded or handloaded equivalent ammunition. The lever action is limited to a maximum cartridge length that will allow the ammunition to feed and or eject reliably. It's probably best to use factory ammunition as a guide for loading your maximum cartridge length. Function testing of course is needed to ensure feeding reliability.
John
