Primers and Shot Start Pressure

GSC recommends always using a magnum primer with GSC drive band bullets and here is the reason: 

The combination of inertia and engraving pressure of a bullet is called the shot start pressure.

Any powder burns differently, depending on the inertia of the weight of the bullet and the engraving pressure that a particular bullet develops.  This burn rate differs within the same cartridge if the shape of the bullet differs or the manufacturer differs.  Also within the same cartridge, the rule of thumb is that one would use a faster burning powder with a lighter bullet.  This is because a lighter bullet generally has a shorter bearing surface and therefore less engraving pressure and friction.  In addition, it has a smaller volume of material that has to be displaced by the rifling and less inertia than a heavier bullet.  The shot start pressure with a lighter bullet would be less than a heavier bullet of similar manufacture. 

The ogive of a standard or grooved bullet contributes most to the engraving force and the length of the shaft does so, to a lesser degree.  As the bullet accelerates, the shaft becomes easier to engrave.  The inertia of a heavy bullet is higher than a light one.  As the weight goes up, the more difficult the bullet is to get into motion.

The shot start pressure determines the rate of acceleration of the powder.  The higher the shot start pressure is, the more the powder burn rate will accelerate and that is why it is required to use a slower powder, the heavier a bullet is. Reduce the shot start pressure and a faster burning powder is needed.

There are two ways to reduce the shot start pressure.  Reduce the weight of the bullet, reduce the engraving force, or do both.

GSC drive band bullets are usually lighter than similar length jacketed lead bullets because the density of copper is less than the density of lead.  The drive bands on GSC bullets have less volume and, combined with the lower weight, this results in a far lower shot start pressure.  The shot start pressure of typical bullets are:
Brass or copper bullet that is on size 9000psi to 6000psi
Brass or copper bullet that is under size 6000psi to 3500psi
Jacketed lead bullet 4000psi to 3300psi
Grooved copper bullet 3500psi to 3000psi
Cast lead bullet 2500 to 2000psi
GSC drive band bullet 1000psi to 650psi

The low shot start pressure of GSC bullets does not always result in a consistent burn acceleration of the powder with a standard primer.  A magnum type primer is hotter than a standard primer and will give far more consistent burn acceleration of the powder. It also follows that, if GSC drive band bullets develop far lower shot start pressure than standard or grooved bullets, they also develop less friction going down the barrel of a rifle.

To verify the shot start pressure of GSC bullets there is a simple test that one can do.  Pressure and speed go together - the more pressure the higher the speed.  Load a GSC drive band bullet and a same weight bullet from another manufacturer with the same components, including the same charge of powder.  Take care to allow similar case volumes with the bullet seated.  Shoot both over a chronograph and record the speed.  The bullet that goes fastest develops the higher pressure and therefore develops the higher shot start pressure and the most friction going down the barrel. 

Remember that all this is applicable within the same cartridge.  Comparing a 110gr bullet in a 270 Winchester with a 110gr bullet in a 270 Weatherby Magnum does not hold water.