The priming in the pan does NOT "FSSST" unless there is a problem with the powder.
The "Fsst" is gas escape from the vent.
Having proved a number of FL barrels there is a "vent pop" from standard breeches as the barrel fires. In this case with a fuse and powder at the vent.
This vent pop is a pressure wave, probably supersonic, exiting the vent.
As I have stated and Pletch has too the "FSSST" is SURELY vent related. UNLESS the prime is wet or extremely large grained.
The worse "FFFSSSTTT" I ever had was at Granger Wy in a match. It was hot and dry as it usually is in the east slope region in summer. I ran a wet patch in the bore and since the barrel was hot did not worry about reloading. The patch was TOO wet. When I seated the ball it pushed wet fouling out the vent into the pan (great thinks I
). I figure what the heck wipe the pan prime heavily. Aim trip the trigger and hold through the long "FFFSSSSSTTTTT" and got the target.
I virtually never get a "FSSST" if the vent is not obstructed the gun fires if the pan does. UNLESS its wet. If the vent is obstructed its a flash. Hangs are invariably flint related unless the vent is very dirty/partly obstructed or the vent is damp and slows the initial light off of the main charge.
I make my own liners and have for years. They are much like a White Lightning with minor changes internally depending on the barrel/breech design.
Ideally the vent looks like this when loaded.
I never pick a vent unless it becomes obstructed.
The reason is obvious.
This the same vent. When freshly installed and I dumped in some powder just to photograph it.
To install the barrel is drilled and counter bored
Then tapped
Magic marker is used to fit everything properly at both the internal and external shoulder (the internal shoulder reduces pressure on the threaded joint by a considerable amount making the vent more secure.
Once fitted properly and snugged down its filed flush.
Careful calculation with drilling will result in a .030" or so "web" between the pan and the counter bore inside the vent.
Dan