A particle accelerator accelerates particles (usually by using and induced force with electromagnets on the particles). Accelerators are usually in a ring-like configuration and the path of the particles can be controlled by altering the induced force field (the containment field if you like).

You can stick a person on a table and place the table in the beam path, hey presto you can shoot someone with a particle accelerator, but why would you want to? If the person is suffering from an intracranial tumour or a brain tumor then open surgery is very difficult. If the particles that you shoot at the person have a high mass (C12 say) then they will travel a distance before they dump their kinetic energy in the surrounding material. By tuning the velocity of these incident particles you can select how far into the patients head they will travel before destroying the matter that surrounds them. This ability in addition to high resolution cat scans enable one to remove deep tumours without opening the skull of the patient.

Sounds far fetched, but it is being done at GSI in Germany, a small particle accelerator in the south of Germany, near Frankfurt. I worked there for the summer, had a great time (didn't get to shoot anyone myself).