Love my USP45 Expert. Wanted to buy a NIB 40 Expert but none available. Have decided to buy a NIB USP40 and modify it instead. Trying to duplicate key features of the Expert. Will install Match trigger (best to send to HKCS?). Sights are no problem. Don’t know if it is possible to install o-ringed barrel. I believe Tactical models have such barrels in the proper length and for that matter I would just prefer to buy Tactical but sadly—I live in CA. Can I install a tactical barrel and have a smith remove the threads? Would it be legal at that point, and would I need any additional mods to approximate the Expert? Any feedback other than, “buy a Glock” will be appreciated.