Not 100% sure but I think non thais can own a pistol if it is registered,bought through the police or documents are submitted through police upon applying to buy and ammo and gun must be locked in separate areas of their property.
It is only able for use of target shooting as the reason , so unless some armed lunatic is trying to shoot someone in your property then a non Thai cannot fire the gun without being in trouble and even a Thai with a registered gun shooting at someone trying to enter their home or shooting at them in their home has to have very good reason for using it .