[douglas]

There are a few major problems with anti proton beam weapons.

First, a stream consisting entirely of negatively charged particles will spread out very quickly if it has any significant density. This problem could easily be solved by making it a stream of anti hydrogen instead.

Second, and much more important, simply throwing antimatter at any sort of shielding system that can easily block purely kinetic weapons without resorting to just putting a big block of material (i.e. armor) in front of them will easily neutralize any stream of antimatter that isn't massive enough to penetrate the shield regardless of whether it's matter or antimatter. In particular, gravitic or any sort of energy or force field shielding will not provide any material for the antimatter to annihilate with. If you can send enough antimatter at high enough velocity to get through the shield, why not spare the expense of antimatter and just throw big rocks instead? You'd be able to throw a whole lot more of them for the same cost if you just used ordinary matter.

The only way to solve this problem is to send some matter along, too. While this solves the problem of how to get the energy release of matter-antimatter annihilation despite the enemy shields, it brings up a whole new problem of how to get it to happen at the target, rather than halfway there where it would dissipate harmlessly. There are two ways I can think of to do this.

First, send two separate streams of matter and antimatter, carefully targetted to intersect at the target. The problem here is that the two streams would have to be EXTREMELY tightly focused and incredibly precisely calibrated for range. Even if you managed to pull this off, the initial energy release as the tips of the streams intersect would almost certainly push much of the remaining material off course to where it would either not annihilate at all or would do so too early.

Second, you can send the containment system right along with the antimatter, set to fail on arrival. This is the only feasible sounding solution I can think of, and it would require improbably cheap, small, and efficient antimatter containment systems.