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
Never change out a component when reloading unless it is published.
The 44 Magnum case is a little longer.
Projectile diameter.
Bullet diameter, case length.
small pistol large pistol magnum pistol small rifle large rifle magnum rifle .50 BMG These are the primers used in all pistol and rifle ammo that is commercially available. Larger, artillery type, ammunition uses much different primers, obviously.
No appreciable differences