If you have a moderately accurate rifle then at distances of about 500 yards and beyond you may see differences in ignition. Or if you are a Tekno Weenie and cannot shoot your rifle without a chronograph, you may see velocity changes due to erratic ignition levels with a given primer. This will be evidenced by a bullet moving out of group or out of velocity spread when you believe it should have been in the group. If you are very good and have a very good rifle then you may be able to see differences at 100 yards on your bullet groups. Some primers are hotter than others. Some loads of powders and bullets like cool primers and some like hot primers.
I have found different primers give you different accuracy levels with the same given bullet and powder charge. Looking back through my reloading journal I see that in July of 02 I was shooting the 160gn Sierra GK in my 7mmRM Sendero. The powder was RL19 and I tried both Remingtons 9.5M and CCI BR2's. I cleaned the barrel before each 3 3-shot test and never allowed the barrel to heat up. The outside temp was cooler than normal that day (I make note of temp/wind and other factors).
I was shooting 61/62/63 gn of powder with each primer. With the R9.5M my groups ran .9035/.6645/.9875 and with the BR2's they ran 1.220/1.115/1.320. While this is only a small sampling and may not be enough data for some, it is/was enough for me. Primers do make a difference. JohnnyK.
"The only thing necessay for the triumph of evil is for good men to do nothing."
A primer is just another componant like a bullet or a powder charge or even a case. Change one componant, and there can be consequences either good or bad. If you have an established load, changing primers can affect accuracy. That said, you can work up a very accurate load with any good primer.
I have my favorite brand of primers as do most who reload, but I think it is more of a ford chevy debate.
I admit that I know just enough to be dangerous.....but dangerous at ever extending distances.