you may wish to go with stainless steel most high pressure applications use stainless now it seems , plumbing supplies won't have it as a rule , try speicalty piping supplies , another sorce might be to bore a piece of bar stock , or a ss bolt !
the old big black powder guns used for waterfowl would slip pipe inside one another to get the desired thickness not sure thats a good idea or not !
Frischkorn Inc may help they are a speicalty piping co.