I agree that it appears to have been split for a fishing sinker.The metric scale shows it to be about .40 caliber which would be a bit large to be buckshot.000(triple ought) the largest common buckshot measures .36(9.14 mm).
I have seen muzzleloading rifles as small as .28 caliber.
There use to be a .410 shotgun buckshot round that had a "punkin ball' load of about 4 or 5 .41 caliber lead shot, but I haven't seen one of them in years. Monty