Heard from a former Bisley shooter Friday that 60% of accuracy in a load is the primers.

True?

He's a former competitive shooter, but I've also sold him a Judge & multiple HIPoints, so his judgement is suspect.  

Primers can make a difference on accuracy of a load - but 60% of accuracy means what? When you try to understand what is being said it makes no sense.

More so than powder charge or bullet consistency?

That is implied by the 60% number.  This was in reference to a top target shooter who would test different lots of primers, and if he found a good lot #, would buy all the primers from that lot he could find.

I think the powder charge consistency was a given, but that primer variability was the remaining variable.  IE, you could have your loads nearly perfect, but if you had a bad lot of primers, there was nothing else that would help.

Yeah, I'm not buying that primers are the biggest factor.  I'd guess the bullet is the most important part you can control, but I don't spend a lot of time milling about in gun shops.

Sounds like he’s saying “if all the factors are accounted for, then the remaining factor is super important!”  

Maybe.  Perhaps the claim is that variations w/in a powder lot, variations w/in a given weight of bullet, and variations w/in a given weight of case only affect the accuracy of the load 40% compared to variations w/in a primer lot.

Anyone test different primer lots for accuracy?

LOL, no. Competition shooters also buy powder in bulk by the lot number for consistency. Primers are less likely to have enough variation between lot number to make as much difference as powder. I'm not going to mention bullets, as we'll assume we're using match grade bullets.

Depending on caliber, for instance, .308, cases capacity makes more difference, which is why the volumetric capacity is measured, or the cases are sorted by weight, and the cases on either end of the spectrum are not used for accuracy, just the cases that fall withing a small spectrum are used. Next would be neck turning to get the pressure exerted on the bullet ( inside ) and on the chamber wall ( outside ) consistent round to round so the release of the projectile is very close.

Now what I think he's saying, but isn't getting across, is that with his load , one brand and type of primer shrinks his groups 60% when nothing else in his load is changed. That is entirely possible, because some primers don't work as well with some powders in some guns.

I have seen half moa reductions or increases between primer brands and types a few times in my own loads. Usually the differences are much less, but for instance, a couple of my 6.8 loads shot half moa better with CCI #41 than with the CCI #400, with the same powder charge ( load dropped 1 grain and worked back up ).

No, it's not true.

Lake City conducted extensive testing years ago that showed that the bullet accounted for 60% of on-target accuracy, powder 30% and case and primer 10%.

....

Cheers.