Tom, I found it a very good dis-assembly video for 1720T, Prod, or even the 1701P. You did, give me an "oh crap" moment when you turned the trigger assy over. I still remember searching for that tiny safety ball on my first time. Then I noticed that your safety had already been removed.
I am interested in what issues you had with the challenger barrel. I had one years ago to try to fit to a Mrod. While TP style was different, it was not in the correct position needed for Mrod, and did not want to have it cut and re-machined. I did think the TP location matched up with the 1720 breech, just needed a custom TP. Was it just a sealing/leaking issue? It also looked like the barrel was shortened, and possible accuracy loss with the choke gone.
If I understand your forward plan, 12FPE with a regged bottle, and will be used for FT. Just some design comments/thoughts to consider. Having the whole tube as a plenum would be great for a very high powered gun, but think you will loose allot of tight Es shots once you come off reg. I do think an in-tube reg will be much better for your needs. Many more tight Es shots. In general, bottle regs are not as precise as the in-tube, and harder to adjust unless you have a test chamber. I really like Huma's, but the Fortitude reg will work and save you about half the cost. The Fortitude reg does seem to regulate well, just a little harder when it comes to plenum. ie might need to drill a vent hole in air tube. You could add the in-tube latter, just have to keep the bottle reg 100-200psi or more, above the in-tube.
I look forward to your follow-up assembly, build, and test video(s).
. Bookmarked!