As well as the assumption that you can only apply one mod to a weapon, when in fact there are 5 mod slots. I'm going to revise my calculations to assume that half of the bases and mods are for melee and the other half are for ranged weapons, then 20% of ranged weapons are energy and 20% are explosive (missile launchers and fat men), and 40% of melee are bladed, 40% are bludgeon, and 20% are punching (powerfists, etc)
10 base bladed melee x (140/5)^5 = 172,103,680
+ equal number of bludgeon melee
5 base punching x (70/5)^5 = 2,689,120
15 base ballistics ranged x (210/5)^5 = 1,960,368,480
5 base energy ranged x (70/5)^5 = 2,689,120
+ equal number of explosive ranged = 2,689,120
If my assumptions are somewhat accurate, then we'd have around 2,312,643,200 possible weapon combinations.
People underestimate the number of 50 base weapons and 700 mods for those weapons even when we consider exclusivity.