The rifle stock was used to bash Nevill largely by striking his face, arms and head with the butt in such manner:
Some people suggest that instead the killer held the rifle by the barrel and wielded it like a sledgehammer. There are several pieces of evidence though that tell us that is not what happened and that it was wilded like in the photo above. The killer helpd the narrow portion of the butt with one hand, the forwgrip with the other and jabbed Nevill with the butt.
I. The stress fractures to the stock
The rifle stock had several stress fractures and a piece that actually sheered off. The stress fractures and piece that sheered off all ran in the same direction. The fractures were all horizonal. Why would the wood fracture horizontally?
Picure walls closing like something out of Indiana Jones. Imagine the rifle stock being in between the walls with the metal buttplate against one wall and the smaller side against another. How will it fracture as the walls compress it? It will fracture the same way the murder weapon's stock did. There will be horizontal fractures as it is squeezed.
The same force will be at work if the butt of the rifle stock is being bashed into something. The stock is being crushed between the person pushing and the object being stuck, so it will be compressed.
If instead the gun were being swung like a sledge hammer then the damage would have been different.
1) the barrel could be bent or broken free from the receiver. The murder weapon has a lightweight barrel that is not likely to hold up if the weapon is furiously wielded like a sledge hammer. Depending on the location of a strike the barrel could actually bend in which case forget about firing it again until a new one is obtained.
The gun breaks down in half. The barrel and foregrip as one unit and the receiver/stock as the other unit.

Note the small section of the barrel that slides inside the receiver. A great deal of force is going to damage it one way or another.
2) The stock attaches to the receiver in a rather simple way. The receiver has a metal projection sticking out at the end:
The wooden stock has a hole to match this and slides on it. The trigger guard is screwed into the receiver on one end and the stock on the other and this is how the stock stays attached. If the gun is used as a sledgehammer with the stock being the hammer part what is the first part of the stock that start to fracture? The wood around the metal projection will start to fracture because the wood is going to be stressed against the metal projection.
Were there any fractures or damage to the area where it slides into the metal projection? No. Yet if the stock were being bent left, right, up or down that is eactly where the stress would be, where it stildes onto that projection.
Since the barrel was not damaged, the stock was not damaged near the porjection and instead was damaged by a motion of being compressed from front to back this tells us the rifle was used to bash Nevill like the photo demonstrates.
II. Lack of cast off
The gun had blood on it and that blood largely got on it when Nevill was being bashing. He had blood on his body and that blood was mainly projected onto the weapon and the killer. When someone swings a blood covered object like a stedge hammer or axe the blood on it is cast off onto other objects. When the object is raised back up blood is cast off onto the ceiling and the wall behind the person swinging the weapon. The ceiling did not have any cast off blood nor did any walls. This indicates the gun was not swung like an axe and confrims that the weapon was used in the striking motions shown the photo above.
III. Nevill's wounds
Nevill's wounds to his arm, face and head are all consistent with being hit by the butt of the rifle.
He had skin ripped off a finger, bruising to his elbow and his forearm had the wounds seen below. The butt of the stock was jabbed into his arm and left gouges. The flat bottom, top or sides of the stock do not make gauges like that. It was made by the butt of the rifle glancing off and digging in.
The buttplate has ridges and can thus gouge but worse look at the sharp corners of the butt

Nevill raised his right arm blocking his face with it. His elbow was facing to his right and his hand to his left. The killer was striking his arm with the butt of the rifle.