Ok, assuming you're trying to avoid NFA rules...
You can't take a rifle and chop it down to handguns specs (barrel under 16" OR overall length under 26", or both). You CAN upgrade a handgun to rifle specs as long as you go "all the way" - 16" or more barrel and 26" overall. And per a recent revised ruling, if you bought the gun as a handgun and took it to rifle specs, you can then go backwards.
Things you have to avoid:
* Barrel under 16" AND a shoulder stock (illegal combination).
* Handgun specs and a forward vertical grip under the barrel. Such a grip is only legal on rifles.
Things that are weird but legal:
* Run a barrel of 16" or more without a shoulder stock, it's still legal just very weird. Once you hit 26" overall length, you can do a forward vertical grip even without a shoulder stock.
* Weirdest of all: 26" overall, no shoulder stock, barrel UNDER 16", forward vertical grip legal. We call this "Franklin Armory" territory:
http://www.franklinarmory.com/PRODUCTS.html
Note the ATF letter, and the specs on this beast...this is one of the weirdest, "edgiest" skirts around the NFA rules ever
.
* Barrel of 16", overall length UNDER 26", no shoulder stock. At that point it's still legally a handgun with a funky long barrel
. No vertical forward grip allowed.
That last is important during a Charger's transition to rifle. You take the handgun grip off, and you have the reciever and barrel in hand. You take the handgun barrel off and put a rifle barrel on. At that point you're still legal - you don't yet have a rifle, you have a weird handgun. Now add the rifle stock, and you have a legal rifle. You don't have to "momentarily break the law" during the transition, which is a big reason BATF was forced to agree that all this was legal.
If you took the handgun grip off while the handgun barrel was still in the reciever and put the rifle stock on the handgun-length barrel, you've just created an illegal combination.