Plenty of builders just use a carbine tube. It's legal. There are a lot of fearmongers waving the "constructive possession" flag but the reality is that the ATF isn't searching homes based on our purchases. About the only case of recent note was one where a shooter had a carbine and pistol disassembled stored in the same gun bag, had his vehicle searched, the gun seized, the cops later assembled them in an illegal configuration and then sent a pic to the ATF.
Did I mention it all started with a road rage incident, one was a fireman, the other a cop? The two tribes generally have issues and it shold be obvious they deliberately created the thing to dime him out. Unfortunately his lawyer was apparently better at estate planning and lost the case, not even challenging the "evidence."
So much for constructive intent, internet lawyers make more of it than exists. Use the tube you prefer, but keep in mind that if it going to be a pistol the overall length is more important being under 26". Over that and it could be interpreted as a 'firearm' which means it may not be considered covered by CCW. I'm not aware of m/any cases involving that, pretty much another internet concern not actually happening in the wild.
Worry more about why you might get stopped and deal with that. As for "constructive intent" anyone who owns a carbine and a pistol would fall under the internet lawyers risk assessment - yet compare that with the shooting population. Most who own pistols also own other AR's so for the most part we are unimpressed the argument has any validity.