After having worked as a home health tech in my early 20's for about 5 years, part of my duties was refilling portable O2 tanks from a transfill station (7-H cylinders plumbed together filling C,D,& E cylinders, 4 at a time,and recording bottle #'s and test dates in a FDA logbook- ask your dad) the size cylinder your talking about is a C tank. It will have a test date stamped into the bottle which is good for 5 years after this date before it needs to be tested again,also because these are approved for transport you should have no troubles as long as it is labled correctly- change the green paint to blue and remove the O2 lable, the cylinder is safe to 2200 psi but the valve for your no2 may not exceed 1800 psi depending on the manufacture- on a side note,many of the bottles sold at Walmart for paintball guns are the same as many no2 systems with only a different lable such as those used by Dynotune