Assuming the wars of the far, far future aren't either fought exclusively by unmanned spacecraft/robots/drones or in ways so incomprehensible to us that comparisons are meaningless, the only foolproof way to increase the power of an infrantryman's weapons are to make the switch to laser and energy weapons, which can produce a theoretically arbitrary (or at least very high) muzzle energy. But the problem with man-portable energy weapons, especially those in excess of a few megajoules or (dare we dream) gigajoules is how to power them. Presumably the people of the future will have perfected the means of creating antimatter cheaply and efficiently to the point where it can be miniaturized and even weaponized.
But is it safe?
How would the soldiers, commanders and generals of the far future prevent their antimatter batteries from turning into antimatter warheads, and how would they prevent their antimatter warheads from turning friendlies into casualties when their Penning traps are breached in an attack by the enemy? Is there a way to render such devices inert in the event of damage to their EM containment fields or does the very nature of antimatter make any device utilizing it for power one stray laser blast away from a bomb?