I own a new, never assembled, forged lower. In order to install a short barrel and pistol buffer tube I must register it as a handgun.  Local law enforcement is unable to determine how to do this due to the fact that I already own the reciever.  First: how do I register a reciever I already own without a permit to purchase a handgun?  Second: I need to verify that it is legal to install a 16 inch barrel on a pistol (like you would a Thompson Contender). Third: Since I also own an AR15 16 " complete rifle would I be able to legally posess an extra short barrel for my pistol or would this be construed as intent to assemble a short barreled rifle (my 16" Bushmaster)? Would the BATF be able to help me register my lower?

First, as long as the lower has never been assembled into anything, you can *poof* make it a pistol lower by saying so.  No shoulder stock is generally allowed, either

Next, as it's a post-ban lower, your assembled pistol must have an empty weight of less than 50 ounces, if the mag is detachable and outside of the pistol grip.  This can't be done with stock parts, so don't bother trying.  Check Olympic Arms' website for a picture of their "OA-98" pistol for an example of what needs to be done to sneak a post-ban AR pistol in under the 50 ounce weight requirement.

You could permanently attach the magazine to get around the weight requirement, or buy some of these new, super-lightweight lowers, uppers, barrels and components to try and make a legal post-ban AR pistol with a detachable mag.  

The only commercial post-ban pistol which makes the weight requirement and is readily available is the Professional Ordnance carbon-fiber pistol.

I am not sure that it will be easy for you to prove that your lower was never a rifle.  The recomendations I had gotten from my local ATF field agent was to purchase a NEW stripped lower directly from the manufacturer (Via my FFL) such that I have an original invoice that clearly states that I have bought a stripped lower!

It was also suggested to fill out the appropriate pistol paperwork to assure no problems with the state laws with purchasing a pistol.  

I was going to write a letter to the ATF's tech branch for a written response to assure that I don't have problems as saying "somebody said" will not necessarily keep you out of jail!

As for a post-ban AR-pistol.  I am of the oppinion that a bolt action where you pull a bolt back and let it fly forward is the best option.  You can still use your mags and it will be the fastest bolt action around!  Another option I have looked at is a single shot system where any semi-auto upper would work as an auto-unloader.  With a single shot.... it's not semi-auto.... so the crime bill does not apply!

Good Luck!

Humm, are you the first owner of this receiver?  Or did you buy it from X individual or X store?  If the later, then it's gonna be difficult.  It's very possible that they lower was written down as a rifle stripped lower when it was originally purchased.....so....is a rifle.