LOL!!!! I just happened to jump on this thread, but the fact that it was 500 something to 30 something strikes me as very funny.
Anyway, I own several guns of the same caliber, and it boils down to one or all of a few simple reasons:
1. Ease of ammo purchasing
2. Different platforms, same caliber (AR, .223 varmint rifle, Mini-14)
3. Coincidence (.30-06 hunting rifle, .30-06 Springfield A3-03)
4. Gift (had one Taurus 99, got another Taurus 99 as a gift)
5. Love of caliber (Hey, I like .357 revolvers, OK?)
6. .22's are cheap and fun to shoot, therefore I own a lot of .22 rifles/handguns.
Overall, I have multiple guns with each of the following calibers: .22, .223, 7.62x39, 7.62x54, .30-06, .357Mag, 9mm, .45 ACP.