A typical musket like a Brown Bess would be similar to a Martini Henry in the damage it would cause. Each of them is firing a bullet of about 500-550 grains, propelling it between 1,000 and 1,300 feet per second. Basically, a heavy bullet travelling kind of slow. It would hit, and tend to lodge inside the body. The damage it would inflict was significant: broken bones, if one was in the way. But also infection- those big bullets tended to introduce lot of scraps of clothing into the wound, and also were lubricated with animal grease- so a lot of microbes could hitch a ride. Before 1870, germ theory of disease was not commonly accepted and neither was sterilization technique ( by Jacob Lister). So, gangrene, sepsis...all sorts of infections could kill a soldier who was only lightly wounded. A surgeon confronted with soldier who had a wounded leg might take off the leg, rather than risk infection. And no surgeon would attempt abdominal surgery- that soldier was considered doomed.

A modern high-powered military rifle would be shooting a lighter bullet much faster- from around 200 to 70 grains 2,700 to 3,000 feet per second, with earlier cartridges like 30-06 being heavier and slower than later ones like .223 Remington. As military leaders realized that it was perhaps more useful to disable a soldier than kill him, by the 20th c. the bullets also became hard tipped, which meant they often would go right through, not expand and expend all their energy within the body. That, and better medicine, has meant that infection is much less of a risk, and wounds can be smaller. Far fewer soldiers would lose arms and legs. However, a high-velocity bullet can still break and shatter bones and cause a lot of tissue damage just from the shock. And of course as machine guns came into use, a soldier could be wounded several times. Lastly, I am not an expert on modern ammunition and can't expound on it, but can say damage from military hard nose ammo is not the same as soft-nose hunting or defense ammo, where the object is not to disable, but to kill.

As for people being blown backward by the impact of a bullet: this is a Hollywood convention. Simple physics: the reaction of a body being hit by a bullet would have to be the same or less than the reaction of the gun and body of the shooter. So, if the shooter is not being knocked over by the recoil, neither is the target going to be knocked over by the bullet.

http://www.civilwarmed.org/surgeons-call/modern/

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