The stocks are all interchangable. So are the barrels (every barrel needs fitting to the reciever).
The real question is: Are the recievers the same? Besides the finish? Are all the parts the same stuff? If they are, then why do they limit what type of barrel can go on the different models?
For example, why can't I get an "Ultra" 24" bull 223 Rem barrel on my Handi Rifle SB2? According to the web site I can only get a 22" bull barrel. There must be a difference in the strength of the recievers. If there's not, then they're loosing sales by limiting our purchases. And if not, then buying a use barrel of the wrong caliber and using it could be dangerous?