You are correct Bob. I have an arsenal of PB tanks....mostly carbon fiber 50's and 68's. I am looking for a more local supplier of the 22ci with 2'' diameter. My thoughts are just start with the ninja 13ci and make my stuff fit, then make the plunge later and order the 22ci from down south. If my math is correct the 13ci volume is larger than my current HPA conversion tube. I have decided to set the idea of a new high priced airgun purchase aside and put the same amount of money into another QB78D HPA regulated(tank) custom setup. If this produces a setup like I am thinking it will then turn my attention to the at44 with the same regulated setup using a 22ci tank instead of the unregulated tube. There is no fun in leaving stuff alone!