I used an Ithaca/SKB Mod.100 in 12 ga. M/F for a number of years shooting seaducks. Shot at least 1000 rounds of # 2's. No perceptible damage to the gun.
Favorite shell for that gun was Remington 2 3/4" Nitro Mag.

I guess at this point I should say that I would not recommend shooting any steel through those M/F chokes.

Dennis