A magnum primer produces a hotter, more intense flame. This is produced by adding a powdered metal, such as aluminum, to the the priming compound. They are used to ignite larger charges or more "hard to light" powders.
Yes, you need to use large rifle magnum primers.
Never change components when reloading.
The .243 IS a large rifle primer cartridge. However, you should stay with standard primers unless a trusted source of information lists a magnum primer for a given load. When you change ANY one component- powder, bullet, primer- go to the starting load and work up. Magnum primers can give unexpected pressure levels with some powders.
Small Pistol Primers are used for small caliber pistols, any of the various 9mm calibers such as the .380, the 9mm luger, the 9x18 makarov, the .38 special, the .32 acp, the .25acp etc. and even some larger calibers such as the .40 S&W. Large caliber pistol primers are used in .45 caliber and larger. Still other calibers, such as the .357 Magnum for instance, require primers known as magnum primers.
Nothing
Nothing
The 44 Magnum case is a little longer.
Bullet diameter, case length.
Projectile diameter.
Never change out a component when reloading unless it is published.
one is an ice cream, one is an animal.
No appreciable differences