Back bottle kind of gets in the way. You cant really use the back bottle as a stock because it will interfere with your mask.
I would reccomend a vertical bottle marker which is also nice because it doesnt have the external airline.
If you want to run a vertical bottle you can use the 3.5 oz tank and then have a stock on the trigger frame..or run a airline from the vertical adapter down to the universal mounting bracket <PPS pumps dont come stock with bottomline holes on their trigger frames> And then mount the tank there that doesnt get in the way.
Back bottle versus vertical bottle makes a huge difference.
If i had a back bottle gun id run an airline from it down to the trigger frame and the mounting bracket and put the tank on the bottom of the frame.