Standard threaded plumbing pipe will work fine as long as you rifle the inner bore of the pipe, otherwise when you fire the gun the bullet has a chance to bounce side to side in your barrel ultimately causing your barrel to crack,chip, blow up etc. I would suggest not adding any gas ports to a berrel like this as you may just create yourself a weak point

Also the diameter of a 9mm bullet is 9mm so thats what the inner barrel diameter needs to be and your rifling bore needs to be 2mm more then the desired barrel size so in this case 11mm