Here's a decent reason: Nevill's lifeless body shifted, no doubt ending up in that awkward, unnatural position, as Bamber was collecting himself after the struggle. He turns and, using his knowledge of that firearm, knowing it can burn skin which he may have experienced himself, burns Nevill to check for a reaction.
Bamber removed the moderator after the attack, so prodding him with the hot muzzle makes perfect sense, as he needed the moderator to cool prior to pocketing it (he probably stuck it in the box as an after thought at a later time) and wouldn't have wanted to put it back on to shoot him again.
Edit: It may be that the moderator was removed once only Nevill was left alive, or Sheila was in a state of incapacitation, but no dead at that point. This also fits with the revisionism, as Bamber stages Sheila's body (and the comically obvious bible placement), he realises the moderator had to be removed because she can't physically reach the trigger from a reverse thumb position (being 5ft nothing and all that).
Sorry, General, gut it isn't a decent reason!
Same answer, if he saw Nevill move, SHOOT HIM!!! Besides, as I was saying, June needed head shots to finish her off so why wouldn't JB have done this to Nevill once he re-loaded anyway?
Nevill had managed to struggle to the kitchen remember! Are we to believe that JB thought a beating would suffice to finish him off? No way, I dont buy that!
Anyway the evidence shows that Nevill was prodded and whacked with the rifle barrel once he reached the kitchen, is this something Bamber would bother doing?
This is another logical argument to dismiss any terrific struggle in the kitchen, I mean, how could a terrific struggle consist of someone being continually prodded and whacked with the rifle?? Are we to believe that JB and his dad were dancing around the kitchen a few meters appart with JB poking and whacking Nevill?? RIDICULOUS RUBBISH!! Nevill was obviously stationary, holding up his one good arm as the killer prodded him!! probably slumped on a chair ready to topple onto the scuttle! The lamp shade was probably smashed at this time too and Nevills watch was hooked by the sights and pulled off!
But no real beating took place at this stage and once Nevill was beyond fighting back Sheila kept an eye on him while she re-loaded the rifle with the bullets spread out beside the phone!
Then Sheila put four bullets into Nevills head to make sure he couldn't get up again, she had learned her lesson with June getting out of bed and struggling, she had to make sure!
The Bible was picked up and moved by Ron Cook, and Sheila was five feet seven I believe!
But no, I dont think you have given a decent reason for burning Nevills back, General, I dont think one exists!