I assume you swapped extensions, or loosened it for some reason to begin with? Or did it shoot loose? If it shot loose, that would be unusual. I dont have the jam nut wrench, but i work on fix barrels pretty regularly for customers. Clean the jam nut and barrel threads of any lube or anti-seize. You can also put a tiny bit of anti-seize on the face of the extension, where it butts up against the jam nut. I use a custom made wrench to tighten the extension against the jam nut. The nut stays put without me holding it with anything. Word of caution, while doing this, you have to make sure you’re maintaining headspace. If you dont have gauges, send it to Q for head-spacing/torquing.