Dekker is preparing to launch Jessica 6 at New York. We can't have that happening now, right? No problem... well it is a big problem, but nothing that a little sabotage can't take care of.
While Guardian HMG ammo isn't so easy to find, I'd still hold onto it... just use it sparingly, and take the microwave pulse gun as a 2nd weapon. You'll find a lot of MPG batteries laying around. If you know an enemy will drop armor when killed, it's better to just use the normal stream and not alt-fire (a microwave pulse). While MPG alt-fire is a nice and splatterific 1-hit kill, any armor normally dropped by that enemy will get blown up with them. If you don't need that armor, never mind.