I'm way late to this .........
I like my 40 ......
I have 2 45ACP , 2 9mm , 2 380 , 1 45 Colts , 1 45 Schofield , a 38/200 , a 38 and a 357 to go with it in autos , singles and doubles .
I've done mountains of looking at apex bullet weights the 38/200/short/Colts NP/etc is dead last . In the numbers world 38/9mm are a dead heat and the 38 carries the day above 140 gr . Below 140 the extra 18,000 psi helps the 9mm a lot .
You can only do so much with an ACP but like it's counterparts in 35 cal it gains to near Colts with the extra 4-8,000 psi allowed and it keeps that edge up to 260 gr . This would make the 45 Quartet also a dead heat at standard loadings .
Load a 38-40 to 18kpsi like the SAA loads instead of the hot loaded 1910 Winchester rifle loads and the difference between it and the 40 up to 170 gr is a pretty small gap . My 175s only mosey along at 940 fps out of a 2" barrel .
Where this goes out the window is when you step out of the nominal loads . Heavy bullets in the old long low pressure cases will put a whooping on the ACP class cartridges all day long . The whooping gets even more pronounced when you put the short cases in a revolver and the biggun's in a closed breach .
The ACPs however gain back for size and capacity . Most 9 mm Revolvers are only 5 shots while even the LCP or Glock 42 are 6 with an 8 shot extended mag for the Glock . The 1911 carries 6-8 and there's an XD that carries 14 rounds of 45ACP while revolvers are 5-6 and even those 45 revolvers have way bulkier clips .
9 vs 38/357 isn't even a fair contest . The tiny compact LC 9 carries 6 and is hardly bigger than the average man's wallet and standard sizes start at 14 rounds .
This brings us back to the 40 .
It's an awkward in between cartridge born as a mag neutured to a Special .
It does fit most of the auto frames . It carries more rounds than the 45 in the same frames but gives up a couple to the 9mm .
Side by side with the 9mm I'll take a 165 gr bullet for bullet over the 125s at 1100 fps all day long . I don't know what the 185s do in the ACP but 250s will go 900 in my Revolvers so there's no contest there , but 12 rounds in a High Power of 170-180 gr @ 950 fps is nothing to sneeze at .
The platforms built around the 40 just like those built around 45 ACP and 9mm deliver first shots in small clusters and double taps within 8-12" @ 25yd with the slide return pulling the muzzle back to POA .
If a bullet misses vitals , fails to open , fails to penetrate or over expands all you have is shock .
If all you have is shock ftlbs matter as much or more than how they are delivered .
Speed might be the answer but if all that speed is only delivering the same ftlb it doesn't matter .