Hi Badenpowell,
I don't know if leakage is the same for all modern made breeches but I suspect yes. It simply would require threads too tight to completely prevent it and that would jam the action with fouling. Nonetheless, it is not a problem for accuracy or shooting comfort. I just wear a hat and I find the gun is very accurate out to 100 yards. I cannot speak to further distances. The fatal weakness of the Ferguson was not fouling or gas leakage. It was that there was so little wood left in the stock at the breech and behind the lock, that the stocks on any surviving examples showing battle wear are broken through the lock area. Too many went out of service for repairs.
dave