I was looking at one of the hk mp5 clone pistols in 9mm. CDNN has one in the current AD for ~$700 anyone have any experience with this model? They call it the "nightscout". Can I swap out the barrel with a longer length? 12" would do nice things to the accuracy/velocity. I understand it would take some gunsmithing, not my first hk parts build. Just are there any legal issues. It is classed and sold as a pistol already so no retracting shoulder stock. Can I reregister this as a sbr and add a shoulder stock? Anyone have any idea? Tax stamp and background check is not a problem. Just need to know if it's possible.