My recommendation would be to get a car and drive north out of Brescia and go up into Gardone and Marcheno, where much of the gunmaking goes on. There will be no need to ask, you'll know it when you get there by the names of the well-known gunmakers you'll see on the signs on both sides of the road. The Beretta factory is fairly monolithic, and straddles that main road through the Val Trompia. If there are some specific gunmakers you want to see, you should check their websites and see if they give directions, since some of them are located on side streets.
My other advice is not to get tunnel vision just looking for gunmakers, it's a beautiful alpine area, more reminiscent of Austria than what you expect in Italy. And, it goes without saying that the food is fantastic. So, I definitely encourage you to make a full day of it.