That should read "about 1 mm thick." So I'll give you that part. However, math doesn't really come into play. In the US the thickness of sheet metal is codified by law to be 1/20th of an inch thick, or 1.27 mm. In practice it varies a little. Hence the about.I'm not certain where you got your...
It would be vicious. There's historical documentation of a shot made with a .50-90 (1872 Sharps buffalo rifle round) at 1500+ yards, but I'd imagine that took several feats. However, the energy of the round itself doesn't exceed that of a .308, in fact it's in the low-middle range of modern...
The 5E item crafting rules start on page 128 of the DMG. It's optional, but it basically boils down to "can you cast the spells?" and "are you high enough level to make an item of this rarity?" The time it takes under these rules is prohibitive unless the players have quite a bit of downtime...
I know comparatively little about muskets but your statements about lower velocity are, generally, correct. Impact area varies wildly, but on average a musket ball is bigger than a modern bullet with significant overlap. However, I know quite a bit about modern firearms and you are seriously...