One of the great myths about using gas checks is that they prevent leading. Well although it is true that a very small amount of leading can come from the base of the bullet, about 98 per cent or more of leading comes from undersize bullets which let the gas escape around the sides of the bullet, melting the sides of the bullet which in turn can cause some really severe leading.
Years ago I had a .44 magnum pistol that leaded like heck and gas checks did absolutely nothing to stop the leading. Even using pure linotype did not stop the leading either. The problem was one of an under size bullet and because the mold would not throw a larger size bullet I had two choices. Use a slower burning power and more of it to obdurate the bullet so that it filled the bore or go to a softer bullet (much, much softer) and keep on using a small amount of fast burning power that I was using for low velocity mid range loads. The soft bullet was able to obdurate with small amount of power and still seal off the chambers and bore.
So if gas checks do not stop leading then why do I use them, because in rifles, not pistols, I have always got much more consistently smaller groups when using gas checks. The gas check prevents the bullets base from being distorted by the hot burning gases. Once the base of the bullet is distorted you can forget consistent accuracy. This base distortion is due to the much higher velocity that I push my cast bullets in rifles to as compared to pistols ( between 1,700 and 1.900 fps. in my rifles) I always seat the bullet within the case neck to prevent the gas check from coming off as the bullet exists the cartridge case and it also prevents any gas cutting of the sides of the bullet. When the bullet is seated down into the powder charge the gas check will only protect the base of the bullet from being distorted by hot gases, not the sides of the bullet.
I generally like to keep my cast rifle bullets up to around 1800 to 1900 fps because when they are loaded down to move slower the wind just plays to much havoc with the accuracy.
I do not use gas checks in pistols as most of the pistols I load for are not the magnum hunting type pistols but standard velocity military pistol calibers.