No experience with a complete Double Star gun, but I used a Double Star barrel and BCG in one I assembled for myself. I used the 16" fluted stainless bull barrel and BCG that Brownell's sells. The price was decent and everything went together well, with a tight fit and no issues.
I've only had that carbine to the range once so far. The first group was larger than an inch, but after that the gun and I settled down and it printed three honest, five shot groups of a hair under an inch using handloads. No malfunctions. The barrel cleaned very easily. Very promising performance so far.