RE: tight groups
Primers can have an effect. . .but what is the effect?I would guess that the primers in an of themselves(brand, shape, color, etc.) had less impact than I'll bet they increased pressures slightly causing theround to perform better. This affect could have been hitting a pressure sweet spot for the gun,seating the lugs better, causing the case to expand more uniformly in the chamber, who knows. Allwe know is it worked and that is good!
I buy my primers by the 1000 and yes, I'm usually guessing, but I've always had good accuracy. Now I'm shooting Federal GMM in my 300 WSM.