The Double Nine line came along to provide a Western Looking alternative to the High Standard Sentinel. The early Longhorns had 9.5" barrels but they later started calling shorter ones Longhorns too. The actions were, like the Sentinels- Modular and the frames, though aluminum, were solid with no side plates. The barrels were pressed in and pinned. The design was by one Harry Seifreid who worked for High Standard then went to Ruger.
Nobody has said so, but it figures that he must have had a hand in designing the Security Six line of revolvers before his retirement. The entire action, like the HS revolvers is a module attached to the trigger guard and inserted into a solid frame.
The HS .22 revovers were accurate and many of them have fired many thousands of rounds without complication. I have heard that the Sentinel frame sometimes cracked at their thinnest point right next to the hammer retaining pin but this might have been because they got stepped on. A friend had a long horn with a loose barrel - probably because somebody applied excessive force in some manner and overstressed the frame/barrel juncture.
One famous user of the long barreled long horn was Earl Standley Gardner, the creator of Perry Mason. Survival in case of a nuclear war was a fun topic in those days and Gardner published some articles in one of the hook and bullet magazines about his "survival" trips in baja california. He would sail down there on his yacht and use his long horn to collect jackrabbits for survival food. Apparently, that part of Mexico was pretty unpoliced in those days.