I'm not versed on the sales competition between S&W's and Colt Trooper guns, but I do know that My Trooper MkIII would not fit (without forcing) into a Bianchi K-Frame holster for a 6" barrel.

Some years ago, I gave My late 70's Colt Trooper MkIII to My son for his first handgun. Although I had wanted a nickeled gun when I bought that MkIII, I didn't have enough funds at the time. I traded My Ruger Security-Six 6" blue, and $130 for that demo-gun, & included was the factory carton, & a new Safariland basket-weave, suede-lined holster.

My blued Trooper with a 6" heavy barrel, would outshoot nearly every handgun that I owned (at that time), sans My Thompson Contender.