makes no fucking sense. if you would do all that work and cut down the rear section and weld in a bushings for the pushpins, cut and reweld a cocking tube, press in a barrel etc. why the fuck would you make a "cough" SP89?
oh, YOU wouldn't. you would make a MP5k. or some kind of "stretch" model as an AOW or PDW(SBR). SP89 was the model denotation of an IMPORTED PISTOL from HK in germany. there would have NEVER been any other version of an SP89 but as the original imported PISTOL. which can LEGALLY become a rifle (SBR) and revert back to a PISTOL. legally.