I have the Sig Ultra 1911 with the 3-inch bull barrel (no barrel bushing). I installed the Clark sear spring and also a new slide stop because the slide was not locking back on the final round. I took it out for a test, and used 10 different mags, and the slide locked back every single time (over 100 rounds). Out of all those rounds, twice the pistol failed. The hammer was back, but the trigger would not release it. A round was in the chamber. Both times, a quick rack of the slide ejected the live round and loaded another, and both times it fired the follow-up round. And, both times the follow-up round was the final round in the magazine. This is a minor mystery. Thoughts?