The first question to ask is: if these weapons are the sort of strength that you suggest. Why is there a universe of normal matter still left?
But then, how realistic is this universe you are writing. If it's a bit of fun OTT fantasy, then go for it, you can always invent any reason for having anything in such worlds. If you want 'explanations' and you really mean what you wrote about the properties of the weapons i.e. a bit harder SF, you may have to scale down the forces and energies.
Ignoring the problems that
@Wayne Mack points out with the black hole fist, regarding it's inertial mass (1) how would anyone be able to pick it up (2) if you mean the mass of a natural black hole - i.e. 1.4 times the mass of the sun - everything would warp towards it and wouldn't see it, as all light around it can't escape - (plus the poor hand of the person putting the glove on - he would in a seperate space in the universe cut off from his enemy over the event horizon)
Possibly you mean a
micro black hole - say a few dozen tons or maybe a bit more? That might work if you also has a suit of power armour that could help the person lift and wield it. Technically they could possibly exist in real life. But then these micro black holes would also technically have extremely short lifetimes - possibly much less than a second. But then you could invent some sort of 'trap' for it that keeps it 'alive' indefintely. Perhaps even let the user ejected it so that it could evaporate and explode (like a core in Fallout4 - if you get the perk!)
...so if we said it was just a regular gauntlet but somehow, via magic, had the force of a small black hole and not its mass. And it simulated the force that a natural black hole mass could give (i.e. mind-boggingly huge). Then even the slightly tap on any surface would instantly disintegrate the matter (via transfer of momentum), Never mind a punch at an enemy, a user would destroy a planet just getting to the battle. Even just moving this gauntlet in the air would probably create a super hot radioactive blast of plasma that would quickly strip the planet of its atmosphere.
So I'd personally go for the micro black hole option with a power suit
Magnetar grende. Do you reall want a grenade with the a magentic field equivalent to a magnetar??? I read recently that the magnetic fields around magnetars are so strong that they elongate all atomic orbitals so drastrically, that
all chemistry up to 1000 km away would cease as we know it. Essentially all atomic structures would break down, organic and biological. Set one of these off and all molecules in the ~418 million cubic kilometres around the grenade would seperate into individual atoms. This seems overkill for a 'hand' grenade - unless you can pinpoint and throw at an enemy a few thousand km away. Again Wayne's advice is sage
White holes - well, the world is your oyster. Some sort of 'wormhole' connectiing the end of the gun to a star. Why not?
Except white holes are theoretical and have never been observed. In fact we don't really know what one would emit (it's not just matter and light, their space-time would be truly weird, I feel) It feels that should be truly stupendous in energy dissapation, which again suggests it's a bit of huge overkill for merely a machine gun.
Overall, it all depends on the type of story you want to write. If it's gods and superheroes flying about the universe, then these sort of god-tier weapons probably could make sense. If you are doing a gritty WW2 in space with real people, it'll strain the incredulity of the reader! What are you going for?
