That doesn't work too well, unless you have its own ord point just for that tank.
Say you put it in internal ord... well you already have 3 gun options, now you'd have to have 6 (1 for every gun option WITH tank, 1 for every gun option WITHOUT tank).
That screws things up here and down the road.
Before everybody gets their panties in a bunch, we need to ask this:
"What is the weight of the empty tank shell?"
If its a measely 50 pounds or some such it's not worth whining about.