Positronium annihilation opportunities are strongly restricted by the requirement to conserve spin. The spin of a photon is 1; and for some reasons two photons are completely unable to have spins in different directions. They can only have parallel spins (total 2) or antiparallel spins (total 0) - never 1, and this prevents a positron with spin parallel to an electron from annihilating to 2 photons. Only 3 photons are possible (also 5 or larger odd numbers) which is very much slower process. Now how about antiproton? The problem here is that antiprotons are said to annihilate normally into pions - but pion spin is 0. No matter how many pions are produced, whether 3, 4, 5 or more, no combination of pions can possibly handle the combined spin of proton and antiproton with parallel spins. So is such annihilation possible?