For some years I have been fixing bullet holes in banjos. I do not charge for this, because the stories behind these incidents are pretty much worth the repairs that I do.
Most bullet holes in banjos are a result of poor aim. As an example, one that I repaired had two holes in it of about the same size, made by the same bullet, which had hit the player's oversize belt buckle and ricochetted back through the banjo head a second time. The shot was probably aimed at the player, since most people can tell intuitively that just shooting the banjo will not stop the player. In cases like this, where the hole or holes are just in the head (of the banjo), I usually do not repair them but just advise leaving them in place as a conversation piece.
If the bullets are of very high caliber, however (more than .50mm), the holes can take a lot away from the tone. This is even more of a factor if your head already has a hole in it from another bullet. If a hole must be fixed, you need a piece of plastic banjo head material (skin will not work, and anyway if you have just about any size hole in a skin head it is virtually impossible that any unenlightened audience will notice a difference in tone). Cut two circles about 3/8" larger than the hole, one for each side, and glue them in place with Weldbond glue. You have to clamp them somehow, for about an hour, and a very large C-clamp will work just fine, unless you happen to have a hammered dulcimer capo, which is even better. The old-time way, of course, is to just stand there and hold them until the glue dries.
Finally, there is the matter of defense. Since it is probably impractical to equip a banjo so that it will return fire, some builders are now experimenting with banjos made entirely out of steel, even the heads. These do not sound very good, but they afford excellent protection for the player and will last many generations. However, lightening is a problem, and a direct hit is likely to solidify all the parts together so that they cannot be replaced, as well as endangering the player. (One old dentured player I met, had his mouth welded shut by a direct lightening strike on his banjo.)
The best defense against both lightening and sharpshooters is to avoid playing in open spaces, near dobro players, on high stages, and in trees.
On the other hand if you are actually fired upon by members of your own band, it is probably best to find some alternative way of life.