Jager22, flame duration and intensity would be the main differnce in simple terms. My experience with magnum primers goes back to the summer of 1966 and load workup for a 6mm Rem. All the scribes were attempting to scare people away from the CCI Primers which were the first in the magnum primers. The CCI250 and H4831 made that 6mm Rem shine and I never looked back. I've read tests that state the CCI Mag were cooler burning in the 20 degree winter weather, but in reality it didn't affect accuracy. To use them or not use them is a personal preference IMO. I can make no blanket statement pro or con. They may raise pressures, but again I use judgment in reducing loads to start and go for the group and don't give a hoot about velocity. I use them in a 45-70Gov't with RE#7 powder and found there is no unburned powder in the barrel as I had with Rem and Win LR primers. Go with your gut feeling and you'll probably do alright.
In adding to Ruger No.3's reply, do keep in mind that the primers aren't interchangable between standard and magnum in a given load. You must work up your load based upon one primer or the other, and not take an established handload, and arbitrarily substitute primers! Although this may seem redundant here for many loaders, we do have many new handloaders on this forum, and our hope is to make this as informative as possible.
Not only will you perhaps run into pressure problems by substitution of magnum and standard primers in loads that are already near the top end of pressure standards, but you'll find uniformity of loads varies greatly in some cases by no more than changing brands of primers, much less type!
To add to Marshall's info, there is only one way to really determine which primer you should use in a particular load, and that is to shoot it over a chronograph and make a direct comparison.
Start with a load (Manuals starting loads will be OK) which is safe and then load several rounds which are identical except for the primer. Ten rounds with each primer, or more, would be best but 5 will usually give a good indication.
In spite of common belief that magnum primers will give higher pressure and velocity, it just isn't necessarily so. I have frequently found lower Standard Deviation and higher velocity with standard primers, even with powders usually considered to require magnum primers.
After finding which primer shows the best consistency I then work up the load. The final test, of course, is accuracy. Use whichever you find gives the smallest groups.
The caution to not change primers in a maximum load is still valid! Again, do your testing to find the most consistant velocity/SD with a load 10% below the maximum shown in the manuals and then work up with that primer. After all this, the bad news is that the most consistant primer may not give the best accuracy.
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