Is it the buffer tube design length?
Could i just take the stock off my rifle and put a short barrel upper on it and have a pistol?
thanks for the help
chris

I am sure someone who has more knowledge then me will tune in but here is what I have got.

1.  I think there is a faQ somewhere in this forumn that addresses this.
2.  You have to have a Lower "designated at purchase" as a pistol if you are going to make a pistol out of it.  That is to say you cannot legally take a rifle and make it into a pistol.   However, later if you choose you can throw a stock on your pistol and make it a rifle.  If you make a rifle out of your pistol, make sure the barrel is longer than 16".   If the barrel is shorter than 16" on a rifle then you have a short barrel rifle that will need to be "stamping/registration" by the government.  
3.  Yes, if you take the stock off of an AR-15 it is a pistol by definition.  However the Ar-15's are designed so they need the internal workings of the stock "the buffer" so people have several options for buffer designs (another faq in this forum).

"Is it the buffer tube design length? "   I do not know what this means.  But if you go the buffer faq thread posted above you can look at the many options for buffer tube lengths.  

Hope this helps.

thanks for the reply that clears it up. just a shame the formality of have to purchase a new reciever just for a pistol.
thanks chris

You can check with RB Percision and see if they have anymore "blemished" lowers. I got one pistol and one rifle for $75.00 ea. (they are still in shipping so I don't have them in hand as of yet, but I will soon and I'll have the pistol built ASAP.)  

Gene

Quoted: Is it the buffer tube design length? Could i just take the stock off my rifle and put a short barrel upper on it and have a pistol? thanks for the help chris

Taking the stock off of your rifle and putting a sub-16" upper on the gun makes it a short barreled rifle.  If you don't have your approved registration, you're a felon.  You MUST have a lower receiver that has NEVER been built into a rifle first.  Once you build it as a pistol, you can switch back and forth.

Read the letter that's tacked above for clarity.

so i can go from pistol to rifle but not the other way? thanks for the tip on RB i'll check it out.
thanks to all the poster
chris