People who are sleeving their guns down, what tube are you using?

Is there a minimal acceptable grade metal..., aluminium, cold seamless steel, stainless?



The reason I ask,

I've read a lot about tuning lately! I'm wanting to put into practice everything I've read and hopefully learned. I've always wanted to do it, but I'm no academic! Once finished I'll be posting a thread about it, showing my finding and failures/successes!

Sleeving is one part of it, I've a dodgy cylinder that can be fixed but it's only salvageable for a project like Sleeving.
Cheers
Andy