Here is a nice little article, from nasa. About 3% of the total moss is converted directly to energy, which is tons.
Containment is kind of a pain in the a-hole though, and there are certain challenges presented as to how one would power the containment, especially in light of the iso stresses from tremendous acceleration. The ship out accelerates the antimatter particle, it floats to the back of it's chamber and, like, boom and stuff!
Other stressors affect this project as well. Basically with too much thrust, if the chassis isn't strong enough, the back end of a ship would end up moving faster than the front.
Lastly, even though this is a good possibility, heat becomes an issue. Whatever material the blast pushes up against to propel the craft has to be able to withstand the pressure and heat.
Ablative engines of any variety are not practical for producing much of anything but propulsion energy in the near to mid future, and certainly wouldn't prove safe, reliable or otherwise practical as a weapon. It's hard enough to hide a yellow cake factory, imagine hiding a Large Hadron Collider.
Oh yeah, here is the CERN page detailing how useless antimatter is for MAKING energy or bombs. It also covers the containment issues, and the fact that having a single am particle collision would NOT destroy the entire planet, or even a small diner for that matter. http://public.web.cern.ch/public/en/spotlight/SpotlightAandD-en.html