I was just about to type the same thing about the tax stamp. You saved my finger! LOLCut the barrel and go to jail for creating a short barreled rifle. To cut the barrel you need to pay a $200 tax and get BATF approval.
Centerfire has them also for $400 but not sure if they have free shipping. I have been eyeing one of these also. Not sure if id rather get a Century Arms like you posted or a Wise Lite mfg. like in the link below. I wish they would just register some of them as pistols and remove the stock. Then we could have short barrels without a fuss.
Sterling 9mm Sporter Rifle Part #RIFLE-STERLING