Whether you run the server locally on your machine or connect to some other server doesn't affect your weapon stats. Likewise, your hardware has no influence whatsoever on the weapon stats (as long as you are able to play with devent framerates that is) ...
If your installation is screwed somehow (which it seems to be the case wrt to your other posts here), the accuracy of your weapons may indeed be influenced negatively. Do you remember your ping you had during this game?
Finally, your brother simply may be much more of a gunslinger than you are
or he runs some very, very, very, very, very, very inappropriate programs if you take my meaning