First of all, fort weapons don't give any skill bonuses so it shouldn't have any aim bonuses anyway. The +30 aim is way too high anyway. If it was that high people would just get it to fight duels just for the bonus.
Second of all, like previous posters said, appearance has no effect in fort battles so why would it have it as a negative bonus if guns even had bonuses? Having it a -10 appearance is nowhere near enough to cancel out the huge +30 aim anyhow.
Lastly, try to be more specific on the type of sniper rifle you're talking about. I think that other posters thought that you were talking about a modern day sniper rifle like a Barret 50 cal or a Intervention Sniper Rifle. Elmyr listed a great suggestion for a sniper rifle of the appropriate game era, if you would of put that as the sniper rifle of your example I wouldn't have a problem with the type of gun.
So I'm in for new guns as long as they are appropriate to the time era and don't give any bonuses. But I don't like the fact that you suggested a gun that give a huge bonus, a negative bonus that makes no sense for a gun, any sort of bonus, and a description of a gun that doesn't seem to fit in with the time era for some people since you weren't specific with the type of gun.