While I realize you're being a joke, but magnets are heavy as fuck, and need batteries and capacitors which, huge surprise, also tend to be heavy as fuck.
Lightening the weapon is not all that useful if the weight just ends up in the ammunition though.
Perhaps light-weight explosive shell/rocket casings could offer some relief there? That could make for a really big boom at little weight, especially if explosive compound and propellant can also be made lighter. If they could be made into shaped charged then an easily carried weapon could do the job of shooting enemies hiding behind walls, which is a job currently best left to medium and heavy machine guns.
I mean, I doubt you'd be carrying too very much ammunition anyway, and, usefully, you can spread out the ammunition without preventing the weapon from being used, as long as the guy with the launcher keeps at least one mag/cylinder/clip on him.
Actually, those wacky Nazis did something like this, called the "Luftfaust". An eight-barreled, 22mm rocket launcher, intended to be a man-portable way of throwing a good number of explodey things at a plane. Not terribly effective, but they were using shitty explosives and inaccurate rockets.
On a completely different topic again, what about the idea of
clip-fed weapons in an advanced setting. Yes, I do mean clips, not magazines. In general, clips seem like an outdated technology, but I can't help but feel a clip-loaded repeating cannon of some sort would be badass.