I want a weapon that can:
- Chew through reinforced concrete like it were popcorn,
- Blast through 4-inch frontal armor plating of tanks like it were papier-mâché in the wind
- Has a high (or continuous) rate of fire, or makes a high rate of fire unnecessary through sheer murderous awesomeness.
- Be man-portable (a grunt can carry it and use it in a firefight).
Initially I was considering railguns, but I'm starting to suspect the combination of iron plasma and the recoil would probably kill even exoskeletoned and full-body armored infantry.
So what will my Death Legions wield? Lasers? Relativistic shard guns? Some sort of particle weapon? Superacid projectors? The relevant criterion is that they be conceivably manportable and not physically impossible (even if not feasible now, and even if heavy enough to say require a powered exoskeleton, but must be possible and plausible -- i.e. no known way to manipulate gravity besides moving heavy objects around).
Edit: Have been asked about Tech level. I'm quite flexible, from near-present to advanced-miniaturized-fusion reactors, maybe even very early antimatter tech. I'd steer clear of Zero-Point, since a mug's worth of empty space energy could boil our planets oceans. Seems like overkill, even for Marines. Generally, the sooner I can have them, the happier I am with the answer, so slight bias towards presently reachable tech.