madsenshooter wrote on Sep 17^th, 2015 at 8:15pm:

How did the flatness of the Federals compare to the other primers?  With equal loads mine were a bit flatter than CCIs.  In my case both were magnum primers.  My use of magnums has been pretty much limited to either small doses of RL19 or large doses of 50BMG powder.  In those cases, with cast, it had the opposite effect you're seeing and tightened my groups.  Maybe because the hotter primer got the powder burning in a more efficient manner.

In my experiment, the Federal 215 magnum primer was the one that gave me the most vertical spread, and printed the highest on the target.  Based on recoil, velocity was increased.  But since I'm using the same powder charge in all my tests, I can't really comment on your loads.

I called Sierra Tech Line today in an attempt to gain some clarity on what my tests revealed.  According to them:
a. I don't need magnum primers.  using them will only increase ES and SD.  The vertical stringing I saw and the printing higher on the target are both signs I'm using "too much primer." 
b. The Federal 215 magnum primer is in fact the worst offender in this regard.  It was designed specifically to ignite Weatherby cartridges and is the only magnum primer that Sierra states should not be exchanged for other primers, but loads with it worked up carefully.  That same caution is not attached to any other brand large rifle magnum primer.

There are a lot of good studies on line comparing different primers to one another.  Two of the best are from Accurate Loading (   (You need to Login) and Accurate Shooter (   (You need to Login).