Modern lead flat nose (LFN) usually loaded, read hand loaded, in a revolver. Need diameter measures with a caliper to the thousands of an inch and weight done on a scale weighing in grains. For example: .357 diameter and 158 grains a reloading manual would show a picture in either a .38 special or .357 magnum. If it measures .429 and weighs 240 grains in this case .429 translates to .44 caliber, I.e. .44 special or magnum. A scale, calipers, and a loading manual will tell you what you need to know.
As other comment above states it does look rather long for caliber so it maybe a lead rifle bullet , I say lead because it doesn't appear to have a copper jacket. If it measures .308 than it could fit a wide variety of .30 caliber rifles and used as a reduced charge/velocity for small game or just plinking to keep in practice.
Don