No, you're right. I always had to loosen my feed tube to screw my tanks in. I had one tank where the on/off knob lined up completely veritcle so I could never use it with my backbottle valve.
The back bottle design is angled downward to prevent just that from happening. Actually you could easily put a 9 oz bottle on it with a shoulder stock and use it just like that.
The first 2 markers have that type of air set up.
http://www.phantomonline.com/paintballguns/primary.htmOk so there you go if it's a pin valve tank your all set if it's a tank with an on off you need to be creative about it, run a bottom line or something.