Something to check out first
If my memory serves me correctly Some early m-12's in 16 guage were chambered at the factory for a 2 9/16" shell and not the later commom 2 3/4" ammo.
The shell lenth is measured after firing so the longer ammo will chamber and when the crimp unfolds into the chamber , the chamber is not long enough so the case expands into the forcing cone,thus causing increased pressures.
I would check the barrel to see if it is stamped 2 3/4" or have someone carefully measure chamber length before you shoot it. It should be fun.
I wish you all the best.
mulespurs