rmkey,

I've measured the group size averages with my 6.5mm loads based upon primers and brass over 1198 measured groups and have found that primers make a difference.

Small Rifle Primers: (Lapua Brass)
CCI BR-4 - best accuracy
CCI 400 - 10.6% larger average group size at 100 yards.
Fed 205M - 47.9 larger average group size at 100 yards.

Large Rifle Primers: (Norma and Hornady Brass - 0.001 difference in average between the two brass types - no measurable difference due to brass)
Fed 210M - 20.6% larger average group size at 100 yards.