What do you believe was the last new calibre to come out that was really filled a void?

The only one I can think of is the .40S&W. I especially think the .17HMR is worthless.

I like the .357SIG, but I don't think it is neccesary.

Judging solely on your basis that it had to bring something new to the table, I would have to vote for the .50 BMG. While the .40 S&W certainly bridges the gap between 9mm and .45 ACP, I feel the void here was less than the gap between the average soldiers 30-06 and say 20mm or 40mm rounds. The .50 gave the infantry a weapon with much greater range, punch and penetration than the standard .30 caliber M1903 while being a more maneuverable package than the 40mm for example (if the big 20 and 40mm canons were in service at the time).

I don't consider the .17 or the .308 in the running. While they may not be bad cartridges, they really don't offer anything drastically different than other cartridges on the market.  

I'd have to rate them in the following order, based solely on what they offered that was new, which wasn't readily matched by another cartridge with similar performance:

1) .50 BMG
2) .40 S&W (stout and wicked...hehe)
3) .223/5.56mm
4) .308 Winchester
5) .17HMR

Of course that's just my opinion and this is a very subjective topic.

-CH

40SW from your choices.

.338 Lapua filling the gap between the .30 Caliber magnums and the 50BMG in presicion rifles.

For me, 270 WSM.  It has a trajectory equal to the 300 Wby or 300 RUM, but with only a bit more kick than the '06.

I am looking forward to the introduction of the 6.8x43, but am afraid that Remington will screw up the marketing.