It’s not Moran. Moriarty is alive and I’ll prove it.
First off, when investigating a possible suicide, detectives often find the gun away from the body. This is because the act of firing and the act of the body going limp, throws the gun away from the body.
Also there is no blood spatter, no bone fragments, no brain particulates. A gun shot like that would have nearly ripped the back of his head off. And the blood pool would be enormous. I know you’re thinking that they couldn’t show that on tv. EXACTLY! They would have zoomed out, shown a close up shot of his hand, the blood pool spreading out to reach it, Sherlock wiping the blood away from his face. (Yes he would have blood spatter there)
In fact, when the gun goes off, Sherlock looks away. He flinches, a perfect reaction to seeing someone “blow their brains out” a foot in front of you. Moriarty risks revealing his hand by holding onto the gun in case Sherlock tried getting a closer look. Moriarty probably even knew that Sherlock was going to fake it, he just wanted to see it happen. The two arch enemies coming back from the dead to resume their war.