Originally Posted by SBruce
Really, just an opinion; no scientific measurements to support, but I see slightly off center flash holes being a problem when the powder is laying on the low side of the case when fired. When the powder is packed full, it shouldn't matter as much.
All the powder lays in the low side of the case when the chamber's horizontally oriented. Gravity always does that.
If the powder density in the case is 100%, then chamber orientation doesn't matter. At any density less than that, there'll always be some space between the top of the case and the powder.
But I don't think that even a case with only 80% powder density will show noticable difference in shot dispersion from flash hole's being anywhere from zero to 100% off center. But there is a difference between the powder's front to back orientation in the case; that's already been proven.
Match grade ammo made in British and Australian cases with Berdan priming didn't seem to have any problems. Loading density was about 90% and both 2- and 3-vent hole versions were used. None of the locak top shots had any concerns about two or three primer flash vent holes all off center in the case head.