We shot 20 gauge Rottweil slugs today in our Mossberg 500 and got pressure signs. After the first shot the action could not be opened. I thought it was a fluke, rapped the action and got it open, and examined the gun. It looked OK. The second shot did the same thing, and the shell was stuck in the chamber.
The gun is in good condition and fires Federal slugs just fine. There are no signs of damage to the gun after firing the Rottweils, but I recommend you avoid them.