A gas check is a shallow guilding metal ("copper") cup that is pressed onto a cast bullet base before sizing and lubricating. The bullet base must be designed for the gas check and have the reduced diameter base to accommodate it.
Gas checks do just that: they "check" or prevent the powder combustion gas from melting or deforming the lead bullet base. The are most commonly used in cast rifle bullets, and to a lesser degree, high velocity cast bullets in magnum calibers such as .357 mag, .41 mag, .44 mag, etc. They do their best work in heavy loads where a large charge of slow-burning powder is needed for the desired velocity. Many handloaders report little or no leading, and excellent accuracy when using plain cast bullets. Using gas checks makes it much easier to achieve those same desired goals of no leading and excellent accuracy.
There is a slight, but still possible hazard to using gas checks: they have been known to come off the bullet base and form a bore obstruction. Firing a subsequent shot with such an obstruction will "ring" and damage the bore.
Using gaschecked bullets makes a BIG difference in loading high velocity rifle lead bullets. It is commonly the only way you can get acceptable velocity and accuracy.
For your proposed 300 gain .45 Colt loads, I suggest first doing extensive load development and casting alloys to get accuracy and no leading. To go to a gas checked bullet design requires a new mold or custom casting.
HTH
John
