Hi Foresterdj,
One turn of the trigger guard handle lowers the screw plug enough to clear the chamber. You place a round ball in the chamber and then fill the space up behind it with powder, screw the plug back up, prime the pan and fire. The ball rest against a shoulder like a modern barrel chamber and the explosion forces the lead into the rifling like a modern bullet. There is no gas escape except a little from the top of the hole because the threads are tapered and they seal more tightly as you close the breech. There is a lot about Ferguson rifles on the internet but as always much of it is incorrect, hyperbole, and superficial.
dave