I think the BATFE letter is pretty clear.
You can install a sixteen inch or longer barrel on your pistol frame and then install a folding rifle type stock, effectively converting the pistol into a carbione length rifle.
You cannot install a barrel shorter than sixteen inches on the frame with the folding stock also in place as this turns the firearm into a short barrel rifle.
You may install a pistol grip and a barrel shorter or longer than 16 inches on the frame effective reconverting the weapon back to the original pistol specifications.
If you T/C Encore was originally purchased as a carbine/rifle, and serial numbered as a carbine/rifle frame you may not convert the rifle to a pistol using a short barrel and a pistol grip as this violates the law concerning manufacture of a pistol from a rifle.
If your Encore frame was originally purchased as a pistol and serial numbered in a block as a pistol intended frame, you may convert it to rifle specifications.
There is no law preventing this.
Onmilo you must have just totaly missed the most important part of the letter:
In the situation you present, the attachment of a folding shoulder stock to a pistol having a barrel length of 16 inches or greater would be lawful as long as the overall length of the resulting firearm is at least 26 inches with the stock fully extended. We caution that, because the configuration you have specified results in the manufacture of a rifle, a subsequent reconfiguration of the firearm to a pistol configuration would result in a weapon made from a rifle, which is a weapon controlled by the National Firearms Act (NFA).
This letter clearly states it is in fact illegal to take your legal pistol, turn it legaly into a carbine, and then remove the parts that turned it into a carbine and make it a pistol once again.
This of course is merely this "John R. Spencer Chief, Firearms Technology Branch's" opinion, and may be trumped by a supreme court decision, but it is in fact the agency given the authority to interprete the law on firearms. So you can be more right and still wrong at the same time.
This interpretation is at odds with both the court case and previous statements by the ATF or BATFE.
However if it is an official letter by the people tasked with interpreting and applying the laws, you could still find yourself charged and arrested by enjoying multiple configurations, which are all themselves legal, but the conversion back from carbine to pistol of a firearm originaly a pistol being illegal.
According to this interpretation simply attaching the parts is manufacture of a rifle, turning the legal pistol into a rifle. You then legaly have a rifle which you manufactured, and can only legaly make things from it that are legal to make from a rifle. Since making a pistol from a rifle is illegal, that means you could not unconvert it.
So they are in fact saying it is legal in all forms. It is however only legal to convert one way. What that means to you as a gun owner is that if there is ever a picture (like the security footage at most ranges) or a witness that proves it was at any time made into a carbine, that documents it in carbine format, it will then be a felony for you to ever be seen or have it in pistol format from then on because that alone will prove you illegaly made an NFA weapon, subjecting you to prison time and felony charges.
