I'm not trying to be ignorant here, just really wondering what makes something sticking out of the back of the receiver a "stock".

Would it be legal to have a tube extension that slides over the buffer tube but is just a cylinder?  Or, what about a two-section tube where the front part is the normal buffer tume and the back part is an empty tube?  I know that there are peple who use the rifle-length tube on pistols and I'm guessing it's to get a little more length-of-pull.  

The reason I ask is that in another thread on shooting stance, one of the suggestions is to tuck the buffer tube into your shoulder like a short stock.  Is there a risk that the Feds will declare all AR pistols to be SBRs just because the tube MIGHT/COULD be used as a stock?

There is a letter from the ATF that one of the guys posted on here at it states that the rifle stock is defined as a device that is used to support the rifle in the pocket of the shoulder. You can add a saddle to the buffer tube because it is not supporting the rifle in your shoulder.

A buffer tube is not a stock
A saddle is not a stock
A butt stock IS a stock

There is no mention of a "stock" in the definition of a rifle.

Gun Control Act Definitions
Rifle

18 U.S.C., § 921(A)(7) and 27 CFR § 478.11

The term “Rifle” means a weapon designed or redesigned, made or remade, and intended to be fired from the shoulder and designed or redesigned and made or remade to use the energy of the explosive in a fixed metallic cartridge to fire only a single projectile through a rifled bore for each single pull of the trigger.

Thanks to all.  I belive I understand all that.  My real question is whether or not it would be legal to add an extension to the buffer tube that is only a tube.  The thread on shooting position recommends pulling the buffer tube into your shoulder to stabilize the weapon.  With a short carbine tube this is really uncomfortable.  My anal retentive brain says that adding an extension would fit the definition of "redesigned ... to be fired from the shoulder" and make it a SBR.

By definition, If you redesign it with the "INTENT" to fire it from the shoulder. It is a rifle.

This is a real gray area.
**Disclaimer: the following information is my interpretation from trying to make sense of this, and may have no real solid foundation in law.**

As far as I know, rifle buffer tubes can be used in pistol builds.  

The way I see it, you can choose to use a carbine length buffer tube, you are good to go, since a carbine buffer tube isn't meant to be fired from the shoulder. If you choose to use a rifle length buffer tube, you are good to go since a rifle buffer tube alone isn't meant to be fired from the shoulder.  If you take a carbine length buffer tube and put something on it to make it longer, you are walking on eggshells. You could argue that it isn't meant to be fired from the shoulder, but some JBT ATF agent might argue that you have made it into a device that is meant to be fired from the shoulder, since you unnecessarily made that thing that sticks out from the back of your firearm longer. YMMV.

My suggestion is to just put a pistol buffer tube on it and submit the paperwork to make it an SBR if you are concerned about shouldering your firearm.

There is a letter to the BATF asking if a cane tip on the end of a receiver extension so the pistol would
stand in a safe would be OK. The answer was yes. If he had asked if that same cane tip would be OK to make
it easier to fire from the shoulder, the answer probably would have been NO. "INTENT" being key.
I think it would be hard to prove intent either way. Unless you posted your intent on the internet,
or told everyone you know your intent.

OMG!  I hope no one from BATFE reads these posts.
Since I personally don't have any intent to modify my pistol, this was a intellectual exercise for me.  When it's time to build a SBR, It'll be done with all the right paperwork.

X-2  

Save $50 a month for 4 months then send in the forms for a SBR. Problem solved. :)

Unless you live in michigan  

Or Washington....

Your personal intent is not the key, if it was then any rifle would be legal with a 10" barrel as long as you don't "intend" to fire it from your shoulder. The intent in the definition is the intent of the design, not a personal action intent.
In your example of the cane tip, the intent of the design in the first example is to allow it stand in a safe, therefore it is ok. The second example shows the design intent of the cane tip to aid firing from the shoulder, that is not ok. Even though the physical design is identical, it is the intent of the design that differs. And even that is an over simplification of the whole thing.
 

I understand what you are trying to say. But a "Rifle" with a 10" barrel is an NFA firearm. It would have to have been made as a pistol to make it ok.

Bad choice of words. What I mean to say is if personal intent to fire from the shoulder was the intent referred to in the definition of a rifle then any firearm with any length barrel (minding the 26"LOA rule), regardless of whether it had a buttstock or not, would be legal as long as there was no intent (personal) to fire it from your shoulder. (designed and intended)
We know this is not true therefore the intent of the design must be the intent referred to, otherwise adding a stock, which is not a feature listed in the rifle definition, would not have any bearing on the classification of the firearm. However the design intent of the buttstock is to enable the weapon to be fired from the shoulder and as such adding one to a firearm fits the definition of the firearm being redesigned and intended (by design intent of the buttstock) to be fired from the shoulder.
 

want the answer too. really want a rifle tube for my build. I guess I am good to go?

yes you can use a full length buffer tube. kinda departs the purpose of manueverability of a pistol but if youwant that type of recoil system, which will make it run smoother by the way, you sure can do that. I've seen them but thought they looked too long, at least for my taste

From the letters thread.