ATF holds that removing the stock from a rifle does not make a pistol... it makes a rifle without a stock. Removing the stock from an SBR would make an SBR without a stock.
The confusion in this area comes in part from the fact that if you have a pistol, you can convert it to a rifle, and back to a pistol, lawfully. You cannot convert a rifle to a pistol, ever, so far as I can tell. You can pay $200 and make a 'weapon made from a rifle', but not a pistol.
So, to the OP, unless you have lots of evidence that your rifle started out as a pistol, I wouldn't go that route.