If I remember, a Patent breech has been around since the late 1700's or something like that.
Instead of the flash hole being directly across from the main charge, which ignites the charge from the side.
The spark must travel up a narrow tunnel on an angle. Then it ignites the bottom of the main charge. And some of the main charge is typically in a cavity at the end of the tunnel. Or bottom of the bore.
You can tell my taking a thick ramrod or the appropriate jag and dropping it down a barrel and seeing how far in it goes.
Then drop a narrower rod down the bore and see if you can get it to drop just a little further in, which would be the cavity.