In Ringo's Legacy of the Aldenata series, there are shoulder weapons that fire tiny self contained cartridges that send very small projos to a significant fraction of c. The propellant is a teensy-bitty dollop of antimatter. I wonder if a buckybomb of an appropriate size could be used as such a propellant. The grav-guns, as they were called, were pure death on the Posleen.
Problem there is anything microscopic, or macroscopic moving even an appreciable fraction of
c in atmosphere will set up some nasty effects, mainly a big burst of gamma and/or x-rays from fusion that heat the surrounding air causing it to expand pretty much in the way a nuclear bomb blast explodes outward from the x-rays making superheated air plasma. And it's moving fastest when fired. Unhealthy for everyone, power armor or not.
Granted, this is a baseball, and 90% of
c...
https://what-if.xkcd.com/1/The Grav Guns were a neat idea, I think Ringo was trying to go for something that was unobtanium high-tech, but was also grounded in reality in some way by not being a "ray gun" by firing a projectile.