Hmmm, a bit of a puzzler. 11 mm is about .41 caliber, but bullets are measured at their widest point to determine caliber, so outside the grooves it would be about .44 caliber (.429") as Breezie said. Does the bullet have any lines running longways on it? That would determine if it is swaged or cast which would make a difference in the I.D. Bullets should have two faint lines from the mold if cast. A swaged bullet won't have those lines. A general area where it was found would help also, such as "near a battle field", or "old camp ground", or even "in my back yard" would certainly help. The problem with bullets is that there are thousands of variations, even dozens within any given caliber. Didn't see anything in "Civil War Projectiles II" that fit although it was similar to some Sharps carbine bullets. I tend to think it is not that old. But I will keep thinking and looking until I arrive at a stronger possiblilty if someone don't get it before I do. Monty