Hello
Nice hunting gun. The gun has no problems. The problem is your snapcaps. The caps are most likely 2 3/4" snapcaps.
Your original chambers are shorter than 2 3/4" You can easily find out by takings some sized fired hulls and cutting off an inch or so of the plastic and close the gun on them, or just shorten the snap caps if that is possible.
That is my opinion, let us know if I am correct.
Have you ever had the chambers measured??
Mike