It's not a vent. It's the hole made by drilling the gas port into the barrel. There would be no other way to get a drill in position to drill a hole going from the front wall of the piston chamber into the bore. Look at a vertical section through of the gas block and it will be obvious. Anyway, there's already a pretty big gas vent right under the front sight - it's about 10x the area of this hole. Also, the bullet has already exited the muzzle by the time the piston gets going good, thereby dropping system pressure to atmospheric.

I would guess the forward hole does not penetrate into the bore. Would be interesting to know the distance on the inside from the front edge of the forward hole to the front wall of the piston chamber, but you're probably not too excited about removing the nut. Kuhnhausen's sketches don't quite give enough dimensions to make the calculations, but it could probably be done pretty well by measuring the dimension from the edge of the gas block to the front edge of the hole and then measuring or guessing at the angle of this hole.