Setting aside the pistol platform, while looking only at the terminal end, to fit the Army bill for harder-hitting:
It is clear by their own data, the current 9mm NATO round does not reach this goal, whereas the previous 45 caliber round did. So no question that the 45ACP would work wonderfully, unless recoil was considered an issue by the Army. So if this were the case, and the Army intends to use ball pistol ammo, where is the data that would either show, 1) that the same 9mm ball projectile fired at the increased velocity of the 357sig, would greatly improve terminal performance, or 2) that the increase in frontal area of the 40 caliber is enough to greatly improve terminal performance when shooting ball. IMO, I believe when limited to ball pistol ammo, I doubt the 357sig would reach the goal, and likely the 40 caliber would still fall short of the previous 45ACP. In addition, when you do consider recoil, I�m not so sure the Army would find the 45ACP to be a great jump in comparison to either the 357sig or the 40 caliber.
What about the projectile itself? Delta may be shooting something other than ball from their pistols, but this probably would not carry over to all of today�s Army. Modern LEO loads with their law enforcement hollow points may not fit the bill either. But what about the new generation of expanding fmj projectiles. They are not hollow points and kind of act like a wall anchor to implode upon itself to increase in diameter on the terminal end. If this were something considered in the mix, then suddenly the 9mm comes back into play. You would then maintain the lower recoil, the magazine capacity, in addition to the harder-hitting.
If it were up to me, every combat soldier would be a battle-hardened Marine and would be issued a railed 1911 chambered 45ACP. But, it�s not up to me and today�s Army runs a wide swath of big to little along the lines of most modern law enforcement agencies. In that world, the trend is for striker-fire for the percentage having difficulty with long trigger strokes, 9mm for the percentage having difficulty for increased recoil, and modular for the cost efficiency and ease of fitting that wide swath. So in reality, the platform chosen by the Army may likely not be a hammer fired gun such as the old school Sig, but may likely be a striker fired gun along the lines of the Glock gen 4 type or M&P.
Best
