Just from looking at it, appears to be a .38. it's been fired, but the bullet is in pretty good condition, so my guess is it traveled a long way and was near terminal velocity when it landed. The hollow base just helps the bullet swell into the rifling grooves tighter. In a former life I hand loaded .38's, and some have the concave base and others don't. Starting in the early 1920's the .38 special revolver was THE police pistol and caliber, and that lasted up until the 80's or 90's, when they started carrying the 9mm semi auto. So your bullet could be 90 years old, accounting for the white patina on the lead.