Personally, for a character that specializes in extremely long-ranged combat, a good sniper rifle is without a doubt the best tool for the job. Even if they have to run and gun, the -1 isn't too bad of a penalty, and if the rifle is modded enough it will be just as deadly at 5 meters as it is at 1000.
I used to give my dedicated sniper a shotgun as a backup weapon, but I found on the rare case she needed to work at close ranges the sniper rifle was still better. There are very few things it isn't at least adequate for, except concealed carry.
And the mental image of a gnome carrying a Barrett on a run-and-gun scenario is just too awesome.