Well it is not a musket ball, it is too small for that. The smallest musket ball would be around .69 caliber, it very well may be a rifle or pistol ball though.
I agree with cutler, comes from the same era as muskets though. Muskets have a large, smooth bore barrel, while American rifles were a smaller bore, and the barrel is "rifled." English
hunting rifles were very large bore like a musket, however they weren't around during the revolutionary war. The English were hunting very large dangerous game in Africa, and also
came here to hunt buffalo. There are historic recorded arguments between English and Americans about who used the most lead, the argument being the large .69 caliber ball brought
the animal down with one shot, while the smaller .50 caliber wounded animals that ran away. There is lots of information on this, but it's not all in one place, it has to be gleaned from
many books on the subject. I've hunted with .50 caliber muzzle loaders, and I tend to agree with the British, larger is better, and I now hunt with a .62 caliber flint lock knock off of a
British hunting rifle. The larger lead ball takes a little more powder to get it going, but it will shoot further, flatter, buck the wind better, and carry more terminal energy to the target.
You might be interested in this chart showing the weights of the various size round balls used for bullets.
32 caliber: (.310) 048g—approx 146 balls/pound
36 caliber: (.360) 071g—-approx 98 balls/pound
40 caliber: (.395) 092g—-approx 76 balls/pound
45 caliber: (.445) 133g—-approx 52 balls/pound
50 caliber: (.498) 180g—-approx 38 balls/pound
54 caliber: (.535) 220g—-approx 32 balls/pound
58 caliber: (.560) 280g—-approx 25 balls/pound
62 caliber: (.610) 341g—-approx 20 balls/pound
69 caliber: (.678) 468g—-approx 15 balls/pound
75 caliber: (.715) 545g—-approx 13 balls/pound
