Tubac----Nice M-H and congrats on having the "nads" to actually shoot it! My guess on the 2 chambers that hang up when you open it are:1) rim thickness may be slightly "off"
2) those 2 chambers may be slightly rougher than
the others--very light honing may be the answer.
I shoot a second model pocket army but it is a 44wcf, or in M-H lingo, a Calibre Wnchester 1873.
These are wonderful old guns with a character all their own, a reproduction of these will never have that!
Good luck with your project and keep us informed on your progress.