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The OP asked about the SEATING DIE getting gummy. Although I agree that using a stepped expander plug (like the Lyman "M" die uses) will make it easier to start and seat lead bullets, the gumminess issue will remain.
The basic problem is that the Magtech bullets are soft-swaged and then dipped in lubricant. So, there is bullet lube all over the bullet, including where it contacts the seating die nose punch. Since lube on a bullet's nose is not going to contact the gun's barrel, there is no harm in removing it with a cloth and some solvent, so long as the lube on the cylindrical section of the bullet is not reduced.
CAST bullets with lube grooves usually do not have any lube on their noses, because of the way it is applied to the grooves (unless the bullet maker is sloppy). So, you might want to use those in the future.
Back to the "M" die. Its basic function it to expand the case mouth with a little step in it so that the open end is maybe a couple thousandths over bullet diameter and the section after the step is a couple of thousandths less than bullet diameter. That makes it easier to set the bullet straight in the case (compared to using an expander that makes a small conical funnel of the case mouth). In either design, the flare or step in the case mouth is removed when the crimp is applied. Also, with the "M" die, the part of the case after the step is usually expanded about one thousandth more than with dies that are designed to be used with jacketed bullets, because the lead bullets are usually made a thousandth larger in diameter than jacketed. So, that will make for less force when seating the lead bullets.
SL1
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