Let's pretend you're standing next to me, and I've got a lasgun. I decide I want to shoot you, and you object (understandably). You're probably not going to let me aim the lasgun at your belly, and might try to slap the barrel away.
Let's pretend you're a psyker. While I'm trying to jam the barrel of my gun into your belly and you're trying to slap it away, you decide to blow up my brain. I object to this (also understandably, I think) and redouble my efforts to jab the barrel of my gun into your soft bits and pull the trigger.
Any time you're right next to your opponent in a firefight (or any other kind of fight, really), things are going to get a little frantic. Go watch the movie Equilibrium for examples of firearms used whilst engaged in melee- I think he uses a rifle at one point, not just pistols, so hopefully it illustrates the point that taking actions in melee that may not be melee attacks still necessitate a degree of physical contact- whether it's a rasslin' match or you trying to take someone's head off with a lasgun while he tries to do the same to you with a pike.