Pistol? 100yds? Remington XP-100 or T/C Contender, scoped either way. 5.56NATO is standard caliber for the XP-100 and the T/C could be had in numerous formats.
At 100yds starting distance, accuracy will beat rate of fire in teh arena of handguns. Either of the above are pretty much rifle accurate at that range, and using rifle calibers. I'd probably go for the T/C simply because it can put more serious rounds downrange easily. Something potent, but not going to be too harsh. .30-30 Winchester would work right well at those ranges, as would some of the 7mm wildcats, and neither would eat your hand.
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