I have to admit, back in the day I read the Dixie catalog, and vented the bolster on my replica 1863 Remington Contract Rifle. I'm not sure why, but did the same thing to my 1861, which I got not too many years ago, I'm not really sure why I did the 1861. However, it hasn't hurt anything...but I think that was the last rifle I'd do that to. On both these rifles, the vent points forward, and up. I sure would not vent a gun if the vent pointed out to the left, the right, or was actually drilled in the barrel. The snail type bolster on my Plain's Pistol certainly presents no intelligent place to drill a vent.
However, what I have noticed is that perhaps it allows some crud to blow out the vent hole, instead of back up the nipple, which can clog it. I may be wrong, but it seems, seems, that the nipple stays cleaner/clear more with the vented rifles, than my one other non vented percussion rifle. ?? So, that might be the real benefit. ?? Just a thought.
The other thing I've noticed is that I can get a slightly hotter load with a vented breech, without the hammer coming back to half cock. I don't mean insanely hotter loads, just that some guns will do that when approaching a good "healthy" load.
Having said that, I've since discovered that using a nipple with a better design will also cure the half-cock blow-back, unless the problem is a weak mainspring.
All things considered, if the rifle(s) is/are properly cleaned and loaded, both my vented and un-vented riles (two vented, one not) are equally reliable. ??