I have been lurking, and studying up here for some time now as I prepare to venture into the Mp5 world and have an NFA question for you all. I recall reading that "once a rifle, always a rifle", but does this still apply when a "pistol" is converted to an SBR? I would like to put together a reverse stretch or an sp89, but would like the option to put an endcap on the gun and still carry it as a pistol when space requires. (we can debate the defensive use of these firearms later). Question is, If I buy it as a "handgun" then register it as an SBR, can I never legally use it as a handgun again, or am I ok? Thanks for the insight!