Laurie, now that we have you, the expert, onboard, what primer size/primer hole combo do you use or recommend for accuracy?
That depends on the cartridge size / charge weight and the powder type. For small precision cartridges typified by the BRs and PPCs charged with fast to medium 'quickness' burn powders and used in ultra precision rifles, small primer brass has been long known (and proven by experimentation) to be superior. Same with small dia. flash-holes. (People long ago experimented with drilling them out in the BRs and found that around 70 thou dia. (ie 11 thou' greater than nominal) performance deteriorated.
I've always wanted somebody to produce some very high quality small flash-hole 222 or 223 Rem match brass to see if it made any improvements to this pair in suitable kit. As I have both small and large flash-hole 6.5 Grendel brass, you could say that I could draw some conclusions from this cartridge, but there are so many other qualitative differences between Lapua and PPU 6.5G, any small difference the flash-holes make would be lost amongst the surrounding 'noise', and my out of the box Howa 'Mini' while satisfying is hardly a true precision job. The Soviets used LR primers in the parent 7.62X39mm M43 round, but that's a military number which might be used in Arctic regions in winter and reliable ignition trumps all else - even if the SKS, AK, RPD etc series of weapons were anywhere near capable of offering the precision needed to see differences. I did try to draw some conclusions from performance of 6.8SPC brass differences - Hornady SP + large F-H vs Rem with both aspects large - but frankly couldn't see any noticeable differences either on the chronograph or paper even in a quarter-MOA capable rifle.
The SP/small F-H formula continues to work well up to and around 6.5X47 Lapua case size and charge weights, ie nudging and sometimes just exceeding 40gn of powders in the medium-fast burn category - Re15 (15.5 too?) Viht N150/550; VarGet/H4350 etc.
Moving up a case-size / charge weight step or two and jumping up capacity to bore [area] ratios brackets, nobody seriously recommends the formula in 30-06, 284 Win, 6.5-284 etc never mind short or full-size magnums. You need a lot more primer energy and a larger flame/gas column with the weight of powder charge and slower burners of 4831 and suchlike classes to obtain reliable ignition and decent ES/SD MV values.
So that leaves the in-betweeners, ie primarily 308 Win and cases based on it, the Creedmoors and suchlike running 42-49gn charge weights. Many have both types of ignition system on offer especially since Peterson Cartridge started up. There are people on this forum who believe that small primer / small F-H brass is a mistake across this entire class as it simply doesn't provide sufficient primer energy to give 100% consistent ignition which is an essential prerequisite consistent performance and high-precision results.
There is a lot to this view, but also a lot of practical experience that says otherwise - eg near 100% of national / international level FTR competitors use SP 308 Win brass. I see some very impressive long-range shooting by people using the 6.5 Creedmoor in custom-built rifles with heavy bullet handloads, heavy charges of mid-slow burning powders / high MVs in tactical / PRS / F-Class disciplines.
Also, I know from my own experience and from talking to others of situations where large primer / large F-H works better with the 40-odd gn powder charge cartridges.
1) powder make / grade. Some powders need more primer energy than others even within ranges from one manufacturer and of the same general type. eg Viht N150 seems to ignite more easily than N140. Some ball powders such as CFE-223 don't work well in SP 308 brass, or don't work at all even. People who've used 243 Win SP brass or 6mm cartridges based on this case say that they have problems with some slower burners such as H1000 - reduced MVs with much increased SDs, classic symptoms of inadequate charge ignition. (Out of interest I bought 50 Peterson SP 243s to reform to the 6SLR and will try Viht N165 in them with 108s - a slow burner but usually easy to ignite. I might also drill the flash-holes out to 2mm too in a spirit of experimentation even if they work well in original form with this powder to see what it does to SDs.)
2) ambient / cartridge temperatures. Cold cartridges need more primer energy and this may push an SP set-up over the edge into poor performance or even outright failure. Again, where the boundaries are is difficult to determine and is affected by powder grade etc. We shoot F-Class year round in the UK and can be out in temperatures on or a little above freezing. I have seen 308 Win Palma SP based rounds start to be affected at this level, but it's again very much powder grade related - ie that's where N140 started to deteriorate, but N150 worked perfectly, and H414 ball powder which should have been a bad choice worked
better (higher MVs and smaller ES/SD) against a control group loaded with same bullets and Lapua LP brass shot in the same winter range session.
3) cartridge case-head strength / ability to handle heavy loads / high pressures. SP brass is always stronger than LP equivalent all other things (such as brass hardness) being equal. FTR competitors routinely use Lapua 308 Palma case combinations that at best would kill same make LP brass in a couple of firings through slack pockets; at worst see retirements from blowing primers. I've never used the original PPU 6.5G cases, but many people on the 65Grendel Forum say they lasted no time at all even with the modest SAAMI pressure 52,000 psi loads. The 6.8s I mentioned are the same - the LP pocket leaves precious little metal around it in this diameter case-head, so R-P brass will have a short life at full pressures.
