My Colt originally came with a 20" A2 barrel on it. I think this is the order in which I changed things. I put a heavier profile blackened stainless steel 16" barrel on it. Then I bought an Eagle Arms flat-top upper and put the 16" barrel on it. Next I put about a 13" long free float tube over the sawed off front sight. That's the upper I have that's 2 or 3 different colors and may have been made from 2 separate pieces. Then I bought the parts it took to have 2 complete uppers that I could use on the Colt lower.
Eventually I bought a DPMS Panther Arms lower with a collapsible stock. I had a new front sight base installed on the 16" barrel and put it on the Colt upper that's finally mated once more with its original lower, and put the DPMS collapsible stock on it. And I put the 20" barrel and flat-top upper on the DPMS lower with the Colt's A2 stock. I sawed off the front sight on that too because the free float tube is about 1/4" too long to fit behind it. I didn't want it between my scope and muzzle anyway.
You put your short barrel in, you take your short barrel out, you put your short barrel in, and you shake it all about. Now I have 2 complete rifles and the uppers have the same size pins as their lowers, and that's what it's all about. I don't need 2 16" barreled rifles, and if I put a scope on the BRN-180 I wouldn't need the other one either. I might be able to sell both for the money I need to build a new rifle, or buy one if Brownells sells complete BRN-180 rifles at some point. Coincidentally, one of the guys I worked with in the army was in charge of Brownells' military and law enforcement division the last I heard. I don't think he could get a friends and family discount for his old direct supervisor. Too bad. I'll just have to pay retail.