I think you touched on what makes this an issue. Specials are one shot kill weapons in their sphere of dominance. But they practically act like primary weapons in the sense that there is no need to conserve their ammo. Take some of the special ammo spawns away and/or significantly extend their respawn time. At the moment, it's easy to get max ammo in the first 60 seconds of the game. OR, you could also add the special ammo going away when you die. That wouldn't solve the issue of ammo camping, but it may be another fun feature to add. Less sniper may also mean more usefulness for the scout rifle, which I never saw in game.
While this is particularly bad with snipers, the same thing happens with shotguns. You never have to think twice about using your shotgun because you're overloaded with ammo constantly.