What do the characters I've found on this old gun mean?
This is a Japanese character. By using Sogou Typewritting (Chinese) you can use this method to see any character that you can't pronounce:

for example, you wanna look at “特”
==type "u" + "niu"（→牛） + "si"（→寺）

——this would give you “特”

so I tried to type "u人布” or "u入布" and could not find this character.
So this might be a japanese character.