Kleptican

This'll probably sound really dumb but I just got a stock phantom with the 10 round horizontal feed tube, backbottle configuration and elected to go with a 3.5oz tank instead of the 12gram with the quick changer. Unfortunately when I try to screw the tank in, the valve on the tank hits the end cap of the feed tube so it won't turn at all. What am I missing? I've been told that a 3.5oz CO2 tank should work just fine on a 10 round feed tube.

Any ideas?

Thanks.