Lets say I have this nice gun which can shoot a certain amount of antimatter. After hearing answers about why that won't work, I added a magical coating to the projectile making sure the antimatter doesn't react upon firing or while hitting air molecules. I recently also learned that matter-antimatter-collision results in gamma bursts, which may be bad for health.
Since I have no idea about the mathematics behind the collision, my question is how much antimatter should be used as a projectile so that the shooter doesn't die of gamma ray overdose shortly after shooting? The difference to the gamma radiation of the target is interesting too. For now take normal humans as shooter and target.
EDIT: To clarify: The goal isn't to kill the target with gamma radiation. The question is how much antimatter I can take before the resulting reaction affects the shooter in a bad way. Gamma radiation is just the byproduct of the collision.
EDIT2: It seems I created some confusion because I revealed to much story background. That's a downer, but I'll keep it in mind for next time. So let's simply image younger brother (30-year-old worker) of the leading antimatter scientist standing in big brother's particle accelerator by accident, about $m$ meters away from the spot where the antimatter is going to hit the matter. How much antimatter will the worker survive?
And I'm very grateful to Michael and Zxu for taking me serious, you guys rock! :-)