Originally posted by KazetNagorra
The mass of elementary particles varies quite a bit, so in antimatter-matter-reactions energy will be converted to mass and mass to energy on quite a large scale. So the final mass of the barrel will depend on what antimatter you're reacting with what matter and what the energy of the antimatter and matter is.
I am not sure but I think I see what you mean -if a positron meets either a neutron then, despite the fact that the positron has only a fraction of the mass of a neutron, does both all the mass of the positron and ALL the mass of the neutron get converted into energy?
Also, I forgot that, because of the nuclear binding energy is different for each isotope for each chemical element, the number of nucleons an atom has is not exactly proportional to its mass (I am assuming here that all the atoms have their full compliment of electrons and are not ionised). So this would mean that the amount of energy released from, say, 4 hydrogen atoms colliding with 4 anti-hydrogen atoms would be a bit different from amount of energy released from, say, 4 hydrogen atoms colliding with a single anti-helium-4 atom.
Ok -but there is a way around this:
Exactly 10kg of antimatter of a type that is the exact antimatter equivalent of the same type of material the barrel is made of (for example, if the barrel is made of pure iron-52 isotope then the antimatter must be pure anti-iron-52 isotope) will annulated almost exactly 10kg of the mass of the barrel (although you will still be left with no barrel).