Your post reminded me of this masters thesis I came across awhile back regarding flash hole size in .223 and .308 brass and the effects of flash hole offset and orientation on precision and pressure.
It really gets down into the rabbit hole but some may find it interesting.
Teaser - the 3mm flash hole seemed best. In other words, larger is better.
Effects of flash hole deviation
Abstract from above linkThe objective of this research is to determine the effect of dimensional and
positional changes of the primer flash hole on the performance of factory-grade rifle
ammunition.
The studied variables were flash hole diameter, offset from center, and
orientation of the offset in the primer pocket.
Cartridge performance was quantified by measuring muzzle velocity, chamber pressure,
and target grouping size (precision).
Five different flash hole diameters were tested for both the Remington .223 and
Winchester .308 calibers: 1.4mm, 2.0mm (the Fiocchi standard), 2.4mm, 2.8mm, and
3.0mm. Each diameter was tested at three offsets: centered (no offset), 0.5mm from
center, and 1.0mm from center. Each of the 0.5mm and 1.0mm offsets were tested at
three orientations: up (12 O’clock), side (3 O’clock), and down (6 O’clock).
Every flash hole was manually drilled and each cartridge hand-loaded in order to conduct
controlled testing of the flash hole variations. Testing took place in two segments with the muzzle
velocity and precision measured at a private range outside Rolla, MO and chamber
pressure and a second muzzle velocity measured at Fiocchi of America (Ozark, MO).
Results showed that muzzle velocity and chamber pressure varied 1-4% from the
control flash hole as hole diameter, offset, and orientation changed. The precision,
particularly in the 3mm diameter and centered flash hole cases resulted in improvements
of up to 28%. Variations in flash hole diameter, offset, and orientation do affect cartridge
performance. Alternate flash hole diameters exist that improve powder ignition
consistently as well as precision. Off centered flash holes increase target grouping size
and result in less consistent muzzle velocity and chamber pressure values.
