Gordon, oft times, there is a deflection backwards, of powder gasses off the bottom of the frizzen- IF the frizzen angles forward far enough, to present a flat or 90degree angle to the barrel of
the frizzen's base - if you get my drift.
Some locks are more prone to this, than others. It had nothing to do with vent position, simply the bottom the frizzen at 90 degrees to your eye-ball. For that reason alone, I have to and do wear glasses when shooting - I have to or I flinch horribly.