Bear in mind that the Krag was not designed for Black Powder.  The .30 USA (as it was originally known) was designed for use with Lafflin and Rand smokeless powder and a 220 grain jacketed round nosed bullet.  40 grains of 4350 and a 220 gr bullet most closely approximates the ballistics of the original round.  I load mine with a CCI 250 magnum primer to ensure that the 4350 burns cleanly, and completely!