I have a buddy that just traded for an AR carbine.  The lower has a "P" stenciled in behind the serial number or model (can't remember and it's not in front of me).  He was told that it is registered as a pistol lower.  But this lower has had a six position receiver extension put back on it.

First is the obvious answer but I've got to ask the quesion (or make the statement).  This cannot have an upper with a barrel length of less than 16 inches put on it because it has had the carbine extension put back on, correct?  Because at that point it will be an SBR and it's not registered for that.

Second, is it legal to put the carbine receiver extension back on it as this is registered as a pistol lower?  And if not, what has to be done to make the a true and legal carbine or rifle again?

Thank you.

First unless Colorado requires pistol registration it is not registered. There is no federal registration and there is no such thing as a "pistol" lower. There are lowers sold as pistol lowers, even stamped PISTOL or PISTOL ONLY but that is just a manufacture/marketing scheme just like the "for military or law enforcement only" Colts.
Unless there is a specific state law to the contrary, any lower can be built into a rifle. A pistol can even be built into a rifle as long as it has a 16" or greater barrel.
The receiver extension is not considered a buttstock so technically as long as you don't have a buttstock that can be ready installed on it then the stock carbine type RE is legal to use on a pistol, it is very ill-advised, but technically legal.  
If it has a buttstock on it and a barrel under 16" then it is a SBR.
So in answer to your question, simply put a 16" or greater barrel on the upper, then put a stock on the lower (in that order) and you have a 100% legal rifle.
 

Thank you for the info.  I'm not sure where this weapon originated from I just know that it ended up in Colorado.  With all the military bases here (five between the Army and Air Force) it could have come from anywhere.  But I now own said weapon.  I'm ripping it apart to get what I need and sell the rest.

The upper and lower are Alexander Arms 6.5 Grendel.  The "P" was professionally etched in after production (not blued).  It's etched in after the model number.  Had a Matech BUIS, KAC M4 RIS and B5 Industries SOPMOD stock mounted on it.  BCG was PSA.  Main reason I wanted it is that it had a Colt M4A1 heavy barrel on it.  Overall condition of the weapon was very good to excellent.  Certain parts were brand new.  Barrel looks to be test fired only.

my only concern would be the P if it is engraved as part of the manufacture's serial number it could be in violation of law.


you cannot change the serial number in any way.



never mind, I mis read it, I see the engraving is after the model number. No worries then.

I read it the same way and was thinking the same thing. I didn't even realize the miss read till ur post