The factory barrels are metric. For a .5x28 barrel, you may want to consider getting one made by Jarvis. They produce an excellent quality barrel. You may want to contact them. They have been sold out for a little while. They are currently in the process of producing a batch of barrels for the P99. I have heard that they will be available by late spring or early summer. I have one of their barrels for a Glock 26. I employ an AAC scorpion on it. IMHO, as an after-market barrel, Jarvis is one of the best around.