I found this bullet in with a bunch of old school reloading stuff that someone gave me. It is headstamped U.M.C. 22-15-60. I took a pic of it next to the 22lr for comparison.
I am not sure of its use miltarily but I know that the 22-15-60 was a chambering in the several Stevens target rifles of the mid-late 1800's. I have heard from a friend who is a Stevens single-shot collector and has a 28-30-120 that the 22-15-60 fouled easily with black powder and thus wasn't as popular as the bigger bores. Like modern smallbores, it shoots straight as long as you keep the bore clean. Very neat find though, thats one I don't have in my cartridge collection. Hold on to that one!
Yeah I have a few oddball bullets in my collection, but this one is pretty cool! It is a centerfire also. Appears to be a small rifle, but could be small pistol primer.