Nevill retreated downstairs while Bamber held the rifle & followed. They wrestled with the rifle resulting in the aga scratch marks and ceiling light smashing. There has never been any suggestion that Nevill ever had control of the rifle.
Once Bamber had control of the rifle again he hit Nevill multiple times with such force it broke the rifle stock, smashed & dislodged Nevill's watch & gave him multiple & horrific arm, face, torso & head injuries.
The only way Nevill could have inflicted injuries to Bamber was if he punched him in the face, once or several times with sufficient force to cause an injury. But Nevill just wrestled with Bamber over the rifle & then defended himself from a barage of violent rifle blows.
It is even less likely Bamber would get a body injury, as he had appropriate clothes on.
As mentioned earlier in this thread, Bamber's clothes would be covered in blood, GSR, oil & may be damaged. But he disposed of them.