There was more than one chambering for the 9.3x72. The "N" designation can mean Normalized, which is what the current S&B and others are more than likely designed for, but there are many 9.3x72s with different chambers. There was a Sauer version, a Nimrod version. The others were differences in the Bullet diameter. Does the case stop due to the base being too wide for for the chamber or does they stop due to the size of the case neck? The bore diameters vary by maker on many older guns. They can be as small as .354 and as large as .375. The Normalized bore is supposed to be .366. With the smaller bore, your chamber neck may be smaller.
These guns are a challenge and mysteries in themselves. That's what makes them so interesting.
In the 12 ga chambers, did you try to see if other brands fit and close (careful with loaded shells!)?
There is the possibility that you have a gun designed for BRASS shells. Very few are, but there are some.