[Piet]

Postino, guys
The experiment that was made, actually demonstrated the result that I think will be obtained. It is quite logical for me, that the firing of a shot wil actually tighten the barrel. I will try to explain the logic as follows.
The rifling is to the right (clockwise) observed from behind the pistol
The barrel screws in to the right (clockwise) observed from the front of the pistol
The barrel screws in to the left (anti clockwise) observed from behind the pistol
When a shot is fired, the bullet at first do not want to rotate, because it is in a non rotational state, and to get it rotating, some external force has to be applied. In order for the bullet to rotate clockwise, the rifling of the barrel apply a force on the bullet in the clockwise direction. The bullet on the other hand tries not to rotate and exerts a force in the opposite direction - which is in the anti- clockwise direction and thereby tightening the barrel.
This rotational force (torque) that is applied by the rifling will be great in the beginning of the bullet's travel because it has to start spinning very fast, but once spinning, the only force that will try to decrease the spinning or rotation, will be the friction in the barrel plus the rifling.
The two forces (the rifling in a clockwise direction) and the bullet's "unwillingness" to rotate in the opposite direction, is not equal to each other. The rifling plus the grip that the person exerts on the pistol overcomes the sluggishness of the bullet to rotate, and is therefore a nett force in the clockwise direction which spins the bullet clockwise.
I can speculate about the rotational force that the hand on the grips have to exert on the pistol in order to keep it from turning in the hand - but I will leave it at that - and first think about it!
Piet