My 38-55 is a Modern 1885 Winchester and the barrel is .375". I inherited a friend's ammo for his BP and old lever actions. His handloaded 38-55's were with .388 bullets and won't chamber in any other 38-55 I have tried.

Before I would invest in a box of 255 grain bullets, I would scrounge up some round balls and slug the barrel from the chamber for 6 or 8 inches. Then buy bullets bases on that end.

Ivan