this may be an elementary question but i'm a bit curious. i have been getting ready to load for an AR/15 using 55gr.fmj's for plinking. in my sierra manual the loads for 55gr using Varget start at 25.1 grains and go up to a max load of 27.2, using a Rem.7.5 primer. in my Hornady book the powder starts at 22.8 and maxes out at 26.4 using a WSR primer.
my question is in some cases does a non magnum primer cause there to be less presure a magmum?

My understanding is that a magnum primer has a hotter flash to ignite a large powder charge or a ball powder that is harder to light. In some cases this hotter flash can lead to higher pressures. But, there can also be cases where a standard primer can have a higher overall pressure then a magnum primer with all other componets being the same. So, my policy si to settle on a group of componets and start well below max charge and slowly work up to an accurate and efficient load. To specifically reply to your question---- in some cases a non magnum primer does cause less pressure in a load than a magnum, however, this is not always true.

<<<my question is in some cases does a non magnum primer cause there to be less presure a magmum?>>>

The answer to your questions is sometimes yes, sometimes no.

The variances in the loads betwen the Sierra and Hornady manuals could also be due to some other variance, maybe not due to primers, in the testing methods used by the two companies.

There are a dozen possible reasons, such as the two companies' bullets have different characteristics even though they weigh the same, the lots of Varget had slightly different burn characteristics, the two test barrels were slightly different, etc....

The point I am trying to make is you have to use the loads listed by the manuals as a general guide and don't take them as perfect recipes for your gun. Whether you use standard or magnum primers, your gun will have pressure characteristics different than the test guns being used by Sierra and Hornady. You'll have to slowly work up the loads for your gun regardless of whether you use the magnum or standard primer.

There has been some informal research done lately that seems to suggest that all other factors being equal, a milder primer flame actually ignites powder more efficiently. I don't recall the author of the series of articles on this subject in Précising Shooting actually going so far as to substitute standard primers for mag. After reading the last installment, I did just that with 10 rounds each way..all other components being identical...

I can say now without a doubt that in my 6mm WSSM there was no consistent difference I could find between rounds loaded with either standard large rifle or magnum primers. They chronographed within a very few fps and the groups were the same. I'm not going to put mags in any standard cartridges to try it that way though.

I had a lenghty conversation with Sierra's support staff on this issue, all things being equal: magnum primers generate higher pressures.

One manufacture's load specs will vary from another's too. Newer specs vary from older publications.

If you want some interesting reading, poke around in a library for some OLD reloading data manuals. See if you can find some stuff from 50's and 60's.

Look at the load specs on some standard calibers. Mfg attorneys have been cranking down the load specs over the years.

Powders have improved/changed over the years too. Load data is all over the internet. Pays to shop around, compare loads.

I stay away from specs posted online by some "good ol' boy" . . . unless it compares to spec published by a reputable mfg.