Robert Le Mal said:for military grade muskets ... and I should say that technically a musket is smooth bored so you wouldn't get any rifling on the bullet, you're looking at .577 or .75 caliber.
So at .577 you're looking at a 3 band Enfield Rifled Musket usually called the pattern 1853 (confusing when they put "rifled" next to musket) or at .75 cal the earlier Brown Bess or it's percussion cap derivative the 1842 model.
How big is the bullet?
The Snider conversion (they took the 3 band Enfield and made a crap breach loader) also fired a .577 bullet.
I may stand corrected but the Bess and the 1842 should have fired a round ball which is close to an ounce of lead if I remember correctly - essentially at .75 cal you have a 12 gauge shotgun. And someone please correct me if I'm wrong it's been a long time since I've handled these things.
The Martini-Henry fired what almost looks like a modern bullet and is .45 calibre soft lead, large & nasty but still black powder.
The bullet could be a lot of things. I doubt it's that early because the Enfield fired a bullet confusingly called a "Minnie Ball" which has 3 bands incised near the base which is hollowed out so that it could engage the rifling when fired and impart spin. Once you've seen a Minnie ball you'll always spot them right off - not like modern bullets at all.
Anyway looks like you're bullet has hit something hard and fragmented? Is it solid lead or is there any trace of copper or brass?
Sorry if that was a bit meandering - was up at 2 am in the low tide.
Robert, you are a wealth of information. I had little if no interest in things ballistic, well up til now anyway.
Nice reply.