A half inch would be a .50 caliber and it would weigh 437 grains according to the conversion chart. Bullets are weighed by grains for some reason. I can't make out the grooves very well and they would offer a big clue as to what era it in which it was fired, which makes a difference in what it might be....clear as mud? The flat base incates it is probably a cartridge bullet, post Civil War I'm thinking. My best guess would be a handloaded .45/70 (not quite a half inch), but it could be any of several buffalo hunting era cartridges such as 50/100, etc. The bullet is a bit light in weight for one of those big boomers though. Monty