[@Ryteb Pymeroce] Anna was adopted by a marine, it wouldn't be too hard for her to get her hands on one. Difficult still, but easier then most. 

Also, in terms of how Anna met the crew, there can be a variety of ways. One they meet her in a tavern somewhere. Two, the captain could have hired her because they needed a gun and she simply decided to hang around. There are other ways, but those are the most probable of the ways.