The outer diameter of the barrel is, for the most part, irrelevent. More important is the internal bore of the barrel.
If you intend shooting First Strikes a bore of around 0.687 is probably the best. A rifled barrel will spin the round adding accuracy to it.
Also make sure any barrel you buy is threaded to fit into your T15, or you can purchase a thread adapter for any type of barrel that will make it fit.
Try Lapco or Hammerhead.
I have a Milsig M17, and the Milsig 400mm Para rifled barrel shoots First Strike consistently. This has an A5 thread, but like I said you could buy an adapter to make it fit your T15.