Gas checks help control lead fouling when cast bullets are driven over about 1500 fps. At lower velocities they are generally superfluous but sometimes enhance accuracy.

Jacketed bullets, especially those with plenty of bearing surface, are usually OK even when a bit under groove diameter (not bore diameter, that's the diameter between lands). If you haven't tried .308" jacketed bullets in your Krag you might be surprised how well they can shoot. That is, after all, the nominal bullet diameter of GI and commercial .30/40 ammunition.

With cast bullets, it's desirable that the rear of the bullet fit the rifle's throat (usually a bit over groove diameter) and critical that the nose is a snug fit between the lands. If the nose is undersize, accuracy can be elusive.

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