I go by this:
FPSC pistol:
400-1500 quads/800-3000 tris 512 or 1024 texture
FPSC larger weapon:
1000-2500 quads
up to 5000 tris 1024 texture
Any other fps engine:
at least 6000 tris
Also it is important to note that when correctly making a current gen weapon, there should be a high poly and low poly weapon. I will not get into the whole process, but most people here don't know that. With a correctly baked normal map (NOT ONE GENERATED FROM THE TEXTURE) you can make a 2000 poly model look like it is a lot, lot more.
Another note: most people agree texture has the largest impact on fps, not poly count. Going slightly over usually will not impact it more than half an fps to 1 fps.