tireman,
Welcome to Graybeards.
Be very careful with that Huntsman with the push in breech plug, that thing is VERY DEADLY! No I am not kidding, those plugs have been known to come back and take out the shooter. Also if you have a misfire, don't open that sucker, they have been known to hangfire and go off when opened. I would not shoot it until you get it fixed. The latch or spring could be broken or weak. The current company will not work on the older Huntsmans, so unless you are handy a gunsmith is in order.
If you are looking to fix it and put some new fiber optic sights on it, I bet you could get a brand new .50 cal Huntsman or Sidekick for a little more.