Yes, the small breathing air 'out of date' bottles are ideal for CO2 /MIG sets.

Filled up from CO2 fire extinguisher , because they only have lever on/off control.

The washing machine seal from Amazon was the last one in the world, saved my life !

Where do the wheels come in?

Thank you Michael, that would make sense alright !

Such a lot to learn !! [Bear with me and be kind ! ]

Reusing old bottles to hold compressed gas is best approached with caution. First problem is old bottles are retired after exceeding their safe number of charge/recharge cycles. The invisible issue in old bottles is the metal flexes and cracks every time the pressure changes during use. After a number of cycles the bottle gradually becomes dangerous. Rather than inspect them with X-rays or whatever, they're dumped before the problem takes hold. The number of safe cycles is calculated and confirmed by checking samples in a lab.

Which gas is in the bottle makes a difference. HP air cylinders are rated up to about 2000psi, which means filling an old one with Carbon Dioxide at about 900psi reduces the stress on the bottle considerably, much less risky than filling an old CO₂ bottle with HP air!

Liquid butane is only about 35psi, and the thin bottles are dodgy for compressed air unless the pressure is kept very low.

Compressed gas bottles are a bit like electric fuses in that they don't do bang the instant the rating is exceeded. A 13A fuse in good condition will pass 100 Amps for a third of a second before blowing, and they survive 30A flowing for about 400 seconds. Although being able to overload fuses repeatedly might seem the practice is acceptable, each incident causes some damage and the fuse may eventually fail unexpectedly at less than 13A. So it is with compressed gas bottles: they are made with a considerable built in safety factor, at least x6 I'd guess, and will take a misleading amount of abuse. Pumping one up a few times doesn't prove anything.

Whereas electric fuses fail safe, gas bottles fail dangerously. Many nasty accidents involving compressed air in torpedos: see HMS Khartoum lost 23rd June 1940. Note the ship sank even though the warhead didn't explode.

No idea if gas bottle steel is easy to shape into wheels. I expect it's tough stuff. Worth a try.

Dave

I think gas bottles tend to get used for wheel rims due to their size, unless you have a free supply of air bottles then it's easy and cheap enough to get hold of ERW tube upto 6" dia, after that not so easy so the Calor gas bottles get cut up.