I'm looking to buy an air tank. All I see are the 100 cu in, the 74 cu ft, and 98 cu ft. Sorry for the ignorant question, but why isn't there anything between 100 cu in and 74 cu ft? I'm looking on Amazon and they have 30 cu ft tanks but are only CE certified and NOT DOT approved. Are these ok to use and fill by myself? Or should I avoid them? I'm hoping to find somethin in the 30 cu ft range so I can bring it small game hunting in case I need a refill. 100 cu in might be too small... 74 cu ft is too big. Any shop that sells 30 cu ft air tanks that's DOT approved? Thanks!