Thanks for your comments.
I have the M1 since 1991, and shot it in IPSC matches since then.
The shotgun allways worked fine, and never had a jam for many years.
And from one day to another...it doesn`t work anymore.
The shotgun has the very same configuration, it had for over these years (13 + 1 rounds, with a 26" barrel). I didn`t add or took off anything.
What`s curious, is that recently I bought a M3, brand new in box. And it started to jam exactly like the old M1.
I`m using really powerfull shells. 36 grams Fiocchi ones, and tried with Remington 00 buck shots.
If I keep it hanging at one side, it works great. The problem is when I hold it tight trying to stand the recoil.
The M1 has fired at least, 50,000 rounds through it.