I've tried the weapons upgrades. At this time, they are mostly worthless. The only ones I've gone from a 3 day purchase to the long term 30 day purchase on are .........
The sub-machine gun pistol replacement. BIG help over the standard issue .45 thats for sure, plus having an auto that fires that fast can be a life saver.
There is one rifle thats a slot 3 upgrade for Medic, Engineer and whatever they call the guy with the floppy hat. Useful upgrade.
If you play on a map that uses helicopters alot, you might consider the Stinger for the Heavy Weapons guy. Having a heat seeker is a big help.
What they really need for the Heavy Weapons man is a M-60 or equivalent so he can lay down some serious firepower. Having just a RPG or a pistol is not much of an option. Also he needs something other than mines for his slot 4. Grenades or SOMETHING. Mines are useless.
Also need something for the standard trooper with a grenade launcher or something to make his firepower heavier than the medic and engineer, and give him more than just the 2 grenades.