+1 to sjy120
Hard to tell from the pic without something in there for perspective but it looks to have a good sectional density.
Measure the diameter as precisely as possible.
5.56mm = .222, .22-250, .223 etc (less likely from the look)
6mm = .243, 6mm ppc
6.5mm = 6.5 swedish, some Japanese military rounds, Italian carcano
Doesn't appear to be any bigger than that from what I can tell
Regards
Dec