Listen carefully peoples. ALL psycannons, no matter who is weilding one, are two handed, even if the model is only holding it in one hand, it still too big to hit someone with and recover for the next attack. Termies only need to swap their stormbolter for a psycannon because their armour gives them a bit more strength whereas normal grey knights need to swap both their weapons to take a psycannon because they dont get the extra strength given by termie armour.