What I did this weekend.. part 2

[John S]

What about the gas left in that's at atmospheric pressure ?
Unless you vacuum a cylinder or use something to displace it then the original product can still stay in there.

[BillTodd]

Well there won't be once the valve has been removed and depressurised.

As John says, there will be gas in the bottle unless something displaces it (even a vacuum pump will not remove all of it) . when air gets in through the open valve (it will get in as the air pressure/temperature changes) the result will be an explosive mix

IT COULD KILL YOU (or some other who's stupid enough to take your advice)

To cut open a gas-bottle (or any fuel tank etc.) safely, do it Irving's way - fill it with water - leave it for a few days to ensure there's no gas trapped in the seams and crevices (and to help remove some of the stink!)