As for your choice of duty calibers, I'm w/ you, because entirely of my experience shooting butcher stock and critters with service pistols. Personally, I'd rather have more bullets in the handle or a New York Reload than a .45, again, just based on killing stuff with the .45 and the 9mm, for example.

I'm not at all saying caliber doesn't matter AT ALL, as the .32 ACP, while being lethal of course, is an amazingly worthless "stopper". That is not to say any service caliber is the Hammer of Thor. Stuck with FMJ's, the 10mm is shocking in is lack of stopping power. I've killed butcher stock and varmints with many service calibers.

But back on topic for a moment;

In reply to lanc above, the US military actually did invest quite a bit of effort in the search for improved personal defense options for pistol carriers. The result was the .30 M1 carbine, tho it "failed" as all shoulder arms do when trying to take the place of a pistol.

As for improvement of the .45 ACP itself, the US military had staggering quantities of ammunition left over from WW1 production and tho they could have developed it further {as was done in the development of the M1 .30-06 round, then "return" with the M2, then early on in WW2 conversion to almost 100% use of AP rounds}, they did not. It would not have been too difficult to do, tho, but the .45 was thought to be heading for obsolescence as indicated by the development of the carbine. In SPITE of the 1917 plan to equip all combat infantrymen with pistols in the previous war.

In spite of the 1940-41 carbine concept, the war itself generated a huge demand {once again...} for pistols. Interestingly, the .45 was heavily criticised by troops and ordnance for many reasons from weight of ammo and pistol, to ballistic performance to bulk of ammunition in shipping, to safety issues involving the SA mechanism, etc. Many do not realize that the call for "something else" was not done by desk jockeys but by many from combat arms units.

Nothing could be done really to improve logistic problems facing troops with the .45 due to ists size and weight and the size and weight of its ammo, of course, and military value of pistols is low enough as lanc says that the overabundance of the guns themselves kept real development of an alternative at arms length for another 2 generations.

But all along, inside ordnance circles, the limitations of the .45 were well-known and potential for ballistic improvement, ditto.