are the 47ci flatlines steel or aluminum?

[Polishpickles451]

Mine has a hydro date of 7 99 and the bottle is made by Catalina cylinders which I always thaught made aluminum products. But it seems to heavy for akuminum. The reason I am asking is becuase if it's aluminum it would need hydro now right? But steel has a 5 year hydro.

[cphilip]

No all metals are five so doesn't matter

I am not sure which on the flatlines but it takes more aluminum to be as strong as steel so often aluminums are bigger and as heaviy if not a little heavier than steel.