The question lacks some rather vital information, so let's clarify, shall we? Is the Scorpion legal to have a buffer tube placed on it, i.e., a non-FUNCTIONAL accessory mounted to the back of the pistol by itself? First things are important - if you are deliberately mounting said buffer tube to deliberately create a Short Barreled Rifle as defined by The National Firearms Act of 1934 and refined by the Gun Control Act of 1968, no, absolutely not legal. If you are mounting said nonfunctional buffer tube for the purpose of mounting a pistol brace in accordance with current federal law, AS IT STANDS AT THIS EXACT MOMENT, I think the answer might be in the affirmative. Now, before you run out and do something rather rash, check YOUR state, county and local laws to ensure that PISTOL BRACES are not specifically outlawed, and/or buffer tube extensions on large format pistols that do not NEED a buffer tube are also legal. Some very anti rights jurisdictions might have some lulu laws waiting in the shadows for you. you must understand that the current federal guidelines for use of a pistol brace and the law are in flux as various legal challenges are not QUITE dead yet - ATFE has until September sometime to appeal the invalidation of their Rule.
But if all you want to do is mount a non functional buffer tube to the back of a Scorpion and NOT use it as a stock or a pistol brace, just hanging there, perhaps as a counter weight, (fill it with sand, maybe?), to the best of my knowledge, (not a lawyer nor do I play one on TV), FEDERALLY I believe it to be legal, but I cannot swear, attest, or affirm beyond the shadow of a doubt in a court of law.
Again, not a lawyer, find a lawyer in your area and put money for actual legal advice, that $200 consulting fee might save you $200,000 in fines.
