[Steve H]

I have just found a "windfall" of primers. A couple thousand small rifle magnum and a couple thousand large pistol magnums. How big of a mistake would it be to use these in non magnum rounds such as .223 and .45 ACP?

[Steve C]

There is no problem with using magnum primers in "regular" loads but you shouldn't just substitute the mag primer in an already developed load as magnum primers will change pressure a bit.

You will need to work the loads back up for accuracy in the rifle so back them down to a start level..

[Floppy_D]

I do regularly. Some people prefer magnum primers in non-magnum loads during colder weather.

[Mal H]

As they say in the Army, if you got 'em, smoke 'em.

Like was implied above, there's no big reason not to use them in "regular" loads. Just be watchful for pressure signs as you work up a load. The max load given in the reloading manuals is no longer max, it could well be overmax.

[Ben Shepherd]

The term "magnum" when you're dealing with primers has NOTHING to do with wether the caliber you're loading is magnum or not. It has to do with the specific powder you are using in that particular load.

[evan price]

Winchester primers for large pistol are marked "For Magnum and Standard Loads" so there is no difference.

Magnum primers burn hotter and throw a longer flame to ignite slow burning low density powders better.

If you are not running right at or above max loads with your ammo, a Magnum primer will give you a slight increase in pressure/velocity but not so much as to cause a great problem, unless you are a bench-rest competition shooter, or running your loads on the edge of safe margins.

Knock down your powder by a tenth of a grain or so and try them out.

[Sunray]

Like Ben and Evan say, a magnum primer has nothing to do with magnum cartridges. The 'magnum' part is a marketing term. Magnum primers burn a bit hotter for a bit longer. They're made to light hard to ignite powders and for extreme cold weather shooting.
However, if your manual says to use 'em, do it. It means that the powder is one of the hard to light powders.
While you don't need 'em for either the .223 or .45 they'll work just fine. As per Steve, you will have to work up the load again when you change any one component.
The CCI 'milspec' primers you hear about(and don't need for a semi-auto milsurp either) are nothing more than magnum primers with a different marketing plan.