So, my final goal of this project is going to be an SBR AR15 in 300blk. My question is this: Can I purchase an off-the-shelf AR lower (tagged either 300blk or Multi Caliber) and build it as an AR pistol (making sure not to purchase any kind of shoulderable stock for it), and shot it as a pistol while waiting on my SBR tax stamp to come back, then slap a stock on it and be ready to go with an SBR?