As common as an old shoe. The best way to cure it for good is to have someone do away with the side ribs all together and install an adjustable hanger on the end. They will come loose as long as you shoot it much even after the repair.
My side ribs have been loose about 5" down on both sides for about 20 years but never got any worse. I'm having my barrels done by Giacomo now but that's only after I noticed the top rib was loose for about the same amount from the breach end. If it wasn't for the top rib getting loose I probably wouldn't have had it redone.
Loose side ribs for the first 6-10" from the breach end is a common problem with all Perazzi's. It's easily fixed. Perazzi changed the design a number of years ago so that this area is now open (split) on all new barrels. This area is covered by the forearm so it's not seen. The side rib ends are now wrapped together so they will not separate.
I have to agree with the above posts recommending doing away with the side ribs altogether and having a barrel hanger installed. This problem is just one of several reasons I went to a MX3 Special combo. My MX8 O/U barrel had to have the side ribs re-soldered twice. The MX3 Special O/U has no side ribs.. Problem solved... Just my experience.. Dan Thome (Trap2)
Dan is well versed in doing away with the sideribs.. and infact custom makes a barrel hangar.. When he's done.. the barrels look just like the MX3 Special barrels.. He's done quite a few for me.. Each and everyone is perfection..
The ultimate test is my 1.36kg MX10 barrel.. fully backbored and choked by Tom Wilkinson.. After years of shooting.. still 100% perfect...