Why not mount the 3.5 in the stock, and have a spring protected line to the front ala Pro-Lite's? Maybe make it a bit longer, something like a handguard. Where the line taps into the valve, put a 1/8" ball valve. When running the tank you'd have the valve open with a dummy 12g in the tube. To switch to 12gs, just close the valve, remove the dummy, and put a live cart in. Maybe have a dummy 3.5 made out of wood to balance the rifle...
Catchya on the Flip Side.....
Emerald Wolf -- Fluff's Sledge Hammer avatar is making me feel old...
