Will magnum primers change group size?

Originally posted by TimSr:

LOL! I could have predicted that, Buck.
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The funny thing to me was the link was for Ask the Experts! and I got a response that was almost certainly copied word for word from a reference, handed out to the so called experts, to limit liability. The "bullet determines whether or not you use a magnum primer" statement led me to believe the only thing "expert" was the title of the link.

Along with Gun Forums, Google and other sources of info, I've generally found that most manufacturers websites are good place to glean info. Some are better than others. To say I was disappointed with the responses I got from Alliant is a given, but like you, it was similar to what I expected, considering the sue happy world we live in today. Still, I really expected some form of response besides, "always follow load recipes and do not deviate from listed components".
 
I tested large rifle and magnum primers in my .30-06 in the cold.

The magnum primers added 12 fps to the muzzle velocity and actually did improve accuracy slightly with 175 grain bullets.
The SD was slightly lower as well.
The temperatures were hovering around 30 deg. F.
I attributed the improvement to a more aggressive initial powder ignition in the cold temperatures.
 
Charles Petty had an article in Handloader in 2006 where a magnum primer, as compared to a mild primer (the mild primer was a Federal 205, but I've forgotten what the magnum was) increased velocity 4.8% (223 Rem, 55 grain V-max, 24 grains RE 10X, 3150 fps raised to 3300 fps). But you don't get that kind of predictability all around. QuickLOAD's author, Hartmut Broemel points out in the program's user manual that there are circumstances under which it makes no difference to use a magnum primer and some under which it can actually lower velocity and pressure, as when unseating a bullet prematurely before a slow powder gets burning well. It's a complicated interaction and testing gives you the only sure answer with your components in your chamber. Just back your powder down about 5% when making a primer change, then use a chronograph to help you adjust the load to get the same velocity you had with the original load before the primer change. You will then be pretty close to the same pressure as long as it's the same powder, case, bullet and bullet seating depth being tested in the same gun.

The main difference a magnum primer makes is it generates a larger volume of gas to provide better starting pressure in a larger volume magnum case. There are other subtle differences in some cases, such as CCI's magnum primer formulation change in 1989 to help ignite the St. Marks spherical propellants better (especially their Western Cannon series of rifle propellants that are sold as Hodgdon and Winchester rifle spherical propellants with different names). They can adjust flame duration and temperature as well as starting pressure and spark showers via primer formulation.

This article describes more about it.
 
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