It was a very interesting and technically successful weapon, but the relatively light weight of the .45 ACP bullet meant that its effective range was very limited: the trajectory curve was like a rainbow.
The modern approach is to load very long, heavy, low-drag bullets (see here:
Untitled Document ). With the benefit of hindsight, something similar could have been done in WW2 by necking-out the .303 case straight to something in the .40-.45 region and loading it with a full-jacket big-game bullet (round-nosed, but still much heavier than the .45 ACP). Any specialist gun firm could have knocked that up quite quickly.