Double adjustable means those shocks now have external valves that allow you to adjust the extension and compression individually, very good thing to have on a drag car
I had a set of single adjustable Strange Engineering on the rear of my old Duster with ladder bars, those work okay on decent tracks but not so good on excellent or poor tracks. I replaced them later with a set of Strange Engr, double adjustable, they where the cats butt
I have no idea what Big Gun valving is, sorry
I have a S/P car now that has a set of single adjustable, extension only Koni coil over front shocks(that have to taken apart to adjust off the car ) that I'm going to replace with a set of new double adjustable with the external valves from one of the better shock companies so I can adjust them at the track easily It does have the newer, better style of double externally adjustable coil over shocks on the rear