Making a murder look like a suicide

I've decided my unfortunate character will "shoot himself" (someone will put a gun in his hand and pull the trigger). The problem is, he has to be incapacitated for this to happen. At first I thought of injecting him with air and shooting him in the same location, but according to my dad they'd find the clot in the autopsy. He could have the gun put to his head and fired, then put in his hand and fired again (as a blank), but making it look right would be a pain in the ass, and blood splatter specialists might figure out that there's no blood on the guy's sleeve.

So how the hell can I make this work? Barbiturates, maybe? Or would forensics figure out that he was already knocked out when he died? I need help. Remember, this is purely hypothetical.

Why a gun? Because it seems right. Why suicide instead of an accident? Trust me, there's a reason. And accidents can sometimes be even harder to fake. There's definitely going to be a clue. Or, well, a couple dozen of them.

Alcohol or drugs sounds like the best bet.

Physically forcing him to blow his own head off? Too risky. There'd almost definitely be signs of a struggle, and the gun could go off prematurely, perhaps even injuring the culprit. Said culprit might have a hell of a time doing such a thing, anyway.