The factors that regulate primer selection are:
Powder burnrate is not constant but accelerates as pressure goes up. The faster pressure increases, the faster the burnrate will accelerate. This acceleration in burn rate, ideally, must be kept between certain levels for most applications. This is the reason why lighter bullets require 'faster' powder and heavier bullets require 'slower' powder.
Two properties of the bullet will determine this increase in burn rate. Together these properties are defined as the shot start pressure:
a) The weight of the bullet determines how readily it will start moving. The heavier the bullet is, the higher the pressure needs to be, to get it into motion.
b) The higher the engraving force of the bullet is when it encounters the rifling, the more pressure it generates. The engraving force of a jacketed lead core or monometal grooved bullet is between 3500psi and 9000psi and shot start pressure is defined as the force required to move the bullet from the seated position, in the cartridge case, to where the bearing surface of the bullet is fully engraved by the rifling of the barrel.
GS Custom drive band bullets are lighter than equivalent length jacketed lead bullets and this slows the burn rate acceleration of the powder. The engraving force of GSC drive band bullets are lower and this also slows the burn rate acceleration of the powder. The engraving force of a GSC drive band bullet is between 650psi and 1000psi.
Because GSC drive band bullets have such a very low engraving force, powder burn rate acceleration is kept within specification by the use of a magnum type primer that will ignite the powder more consistently and burn hotter and for longer. Faster powder is used because of the lower weight for length (specific gravity) of copper as opposed to jacketed lead.
GS Custom Bullets, situated in Port Elizabeth on the East Coast of South Africa, manufactures solid copper, turned, monolithic bullets for hunting and sport shooting. These bullets are used by hunters on several continents, hunting from the smallest of antelope to the largest of dangerous game, using the smooth HP bullet, as well as the more popular HV, FN and SP bullets with the patented drive band concept. GSC bullets are configured for the highest possible ballistic coefficients. SP bullets are mainly used for sport shooting. All GS Custom Bullets are coated.