Richard,
I just did a d/t on my tracker II 20 ga for a scope rail and here is what I can tell you.
The factory scope rail holes on the handi barrels I measured are approx. .090" deep
The 20 ga chamber wall on my barrel is about .120" thick, so 0.03" of metal left after the d/t, if you are paying attention.
The CF rifle barrel that I practiced on, piece of barrel from one of my chop jobs, and the 20 ga barrel drill very easily with a high quality bit.
Go really slow unless you like trying to remove broken taps from small holes. I got lucky and didn't break one on the 20 ga but there is always next time!
IF it where my barrel I would determine the wall thickness where you need to d/t and then make a decision if it is a go or not. I personally wouldn't d/t a barrel unless all the holes are "blind", not through the barrel wall.
BB