OzFclass

I'll make a proper report with supporting data and targets when I've properly finished testing. But here's my as yet unscientific take on tuning loads with same elements of load for my current straight 284 barrel (powder 51.0 2209, 180 Hybrids 23 thous from lands, CCI BR2s seated w 5 thous crush [thanks Rod], no neck lube) with varying amounts of flash-hole expansion. I tested no expansion (aprox 2mm), 2.15 mm, 2.25mm and 2.5mm only. Without extensive-enough trials, I did see enough to convince me that the 2.25mm flash-hole diameter provided a sweet spot for my particular setup. From achieving consistent five shot groups of .28 moa with three groups of 5 shots) the 2.15 drilled sample shot slightly better or the same; the 2.25 group averaged just under .25 MOA once tuned (barrel tuner only) over three groups of 5 shots, with best group scraping under .20 MOA. Each condition required barrel tuner change to find tune. I should have spent more time with the 2.5mm drilled sample, but found it trickier, and hard to get motivated for following 2.25 success. I didn't try greater diameter flash holes, and may not try for the time being with this barrel, as it would be nice to preserve some of its life for future matches. I'll probably go back with another barrel and accumulate enough data to be more conclusive early in the new year. But I'm satisfied that, given my ammo and my rifle/barrel, there is indeed a small amount of improvement in both ES/SD and accuracy. Slight increase in speed with each increment of flash-hole enlargement. Hopefully that is enough to stimulate experimentation by others.

Merry Christmas to everyone, and hope Santa brings you all something manufactured by AMP, Area 419, or Supertrickler.

gday john, i found this thread on accurate shooter. I didnt watch it but the title says it all. Though you might be intereested in it if you havent already seen it. Cheers
https://forum.accurateshooter.com/threa ... t-38520544

For what's it's worth I did some testing between Alpha and Lapua SRP 308 brass which have large and small flash holes respectively.

I trialed 2209 and 2208 with BR4 and 450s. There was no significant different in velocity, ES or the pressure that the primers would withstand between combinations which came as a surprise. I was fully expecting a 450 primer with 2209 and a large flash hole to show a higher velocity and better ES but as both were virtually identical to the small flash hole and BR4.

Care is needed when playing with flash hole diameter. Increasing from2mm to2.5mm will raise the primer pocket back loading ( not pressure)by approx 60 per cent. and could cause the firing pin to pierce the primer.

HI Barry. I'll take that into account. At first thought your idea seemed backwards to my illogical mind - think shrinking diameter down to next to nothing - wouldn't that trap the primer percussion and cause back loading of primer to firing pin? But I now realise that you are talking about the big bang of powder charge blowing back through a bigger and bigger access way to the primer. Gotcha.