Nope.

I'd strongly suggest that you use an existing component rated for the required pressure, or get something designed and fabricated by someone with experience doing pressure vessels. As I'm sure you know (I'm guessing a few of your robots are full pressure?), 750psi gas has the potential to be extremely dangerous.

Incidentally, the design you suggested (I'm assuming the end heads to be flat from you just describing the ends being welded up, they are also assumed to be the same thickness as the tubing) has very little margin for safety whatsoever (750psi will be ~70% of burst pressure according to some quick scribblings) and wouldn't be usable.