Imagine this: The battle of Leipzig/Waterloo/Gettysburg is in full motion. The scene shows the command of the French/English/Union/whatever army. Cannonballs impacting left and right, suddenly the faithful aide to the general/member of the staff, whose funny quips won the hearts of the audience, jerks and falls to the ground, as if struck to death. But, oh luck, a smile appears on his face and he produces from inside his jacket a tin of snuff tobacco ('his fathers blend')/a golden pocketwatch ('a present from his loving wife') or something like that, with a visible dent where it deflected the bullet which otherwise would have pierced his heart.
How feasible is this staple of a lot of movies about historical battles? Wouldn't the bullets and musketballs of that era be powerful enough to pierce such an object, made out of metal? What would it take to realistically stop the bullet, like modern ballistic/bulletproof vests do?
It is MORE than likely that a pocket watch could stop a musket ball. The odd thing is that, in the heat of battle, sometimes soldiers wouldn't fully load their muskets.
So, during battle, soldiers would often stand in a line (open order fighting was coming into popular usage but not as common as line units). The act of physically standing side by side means that you will have a hard time pouring ALL of the gunpowder into your barrel because the soldiers beside you are also doing the same thing, thus you might knock each other and powder could just fall onto the ground.
So, a musket isn't as certain a show as say a contemporary firearm. There are even stories where French cavalry would fold their overcoats in ridges and at the end of a battle, they would unfurl their coats to a literal shower of balls falling onto the floor.
Combine the poor reloading with the effective reloading, a musket only has an effective range of eighty to a hundred meters. And on top of that, not all gunpowder is created equally, there might be imperfections in one cartridge that wouldn't be present in another. Overall, muskets are temperamental and problematic but the most effective in general efficiency.
This is of course range dependant but a muzzle loader is a scary weapon up close. A common Brown Bess service load was a .69 caliber ball with 100 grains of powder (Lyman Black Powder Handbook) which should give you a ball that comes out at around 1000 feet per second with a whopping 1200 ft.lbs. (I seem to recall early models of the Brown Bess having even heavier bullets)
For reference, a normal .44 magnum pistol round has about 1000 ft.lbs. The main problem with muskets is not that they're not leathal at significant range, rather, they become too inaccurate. A slow and heavy projectile actually retains energy relatively well but the bullet experiences significant drop off. The Brown Bess, being without a rear sight, would be very difficult to aim at great range (which in turn would rob volleys of their intended effect).
Of course, a stray bullet could technically be stopped by a small object in a persons pocket but it would have to be at the extreme end of the range and even then it would have to be a very hard object.
Black Powder weapons have a relatively slow muzzle velocity and fire a soft lead round, so its very plausible that a thick metal watch would stop one.
Here is an example from the Civil War museum in Philadelphia http://civilwarmuseumphila.org/collection