To me, this speaks to a longer term sidearm that can start as a 9x19 and potentially got to other calibers should the situation dictate a change.
Delta went to .40's a few years back in hopes of better performance in the field. They were carrying some hi-cap STI's in .40 S&W, but they dropped it due to difficulty in re-supply when abroad. If they do indeed go to a pistol that can be converted to different cartridges, then they really need to address the supply issue.
But if they went with something truly modular (like what they're looking at), then I don't see any reason why they couldn't switch all the way up to .45 ACP; which would take care of the supply issue.
Back in the '70's and '80's the US Army did some really neat stuff with the .45 ACP to improve barrier performance. If we stuck with just 9mm and .45 ACP and perhaps introduced one of those higher performance .45 ACP rounds, we'd probably be in good shape with minimal impact to our supply lines.
