I have a 3 Beretta 686 over and under shotguns. The 20 ga and the 28 ga are the sporting models with the "B-Fast" adjust able comb, 30" and the .410 is the field model, 28" standard stock. They have performed well over many years of skeet shooting with just barrel cleaning and receiver lube. It's time to remove the stocks and clean the trigger assy and internal receiver works.
After removing the butt pads the .410 field appears easy , just an allen bolt must be metric, what size?,
The 20 and 28 look a little different (with BFast) , a mounting screw for the comb passes through through the access hole so obviously that must be removed, and then it appears that hex bolt head in the middle of a steel bushing holds the stock on. What size is this socket? It looks like it would need to be a thin wall socket or is there a special tool?
Help me out guys.

The Allen wrench is 6 mill .Steve!

A friend of mine mentioned that he had bought a stock tool many years ago from Beretta. He loaned it to me and it worked fine. I did have to remove the adj comb bracket though. The tool is nothing more than a 8 inch long 6mm tee handle allen wrench.
Problem solved.

I'm going on memory here but I believe the hex screw has a 13mm hex. BTW, my 686 is equipped with an adjustable comb stock for an older 1980's vintage 682 Gold E. It's old enough to have the original Beretta comb hardware and I'm not going to pull that off the stock to get at that screw. Because one defect of the original system is that the only way to repeat a setting is with calipers and a bit of luck. Note, one day soon I plan on replacing that old hardware with a Graco kit that will drop right into the inletting for that old system.