I don't know about 9mm, but the .45 ACP 185 grain Critical Defense load performs well in a 3.3" bbl (XDS).
That said, I carry Corbon 115 grain +P DPX in PM9, which has a 3" bbl. The Shield has a 3.1" bbl. This is just my opinion....but the advantage that a 9mm counts on is speed, because it is a much smaller bullet than a .45, and perhaps half the mass. But a 3.1 barrel doesn't allow the same velocity as a duty-sized barrel, so it would be my guess that a standard pressure load in a short barrel can't develop enough steam to be as stout as you want it to be, hence +P for short barrels.
Hornady posts ballistic figures for both 5" and 3" barrels for the 185 grain .45 load (
LINKEY), but for the 9mm they don't do that. They offer a 100 grain and 115 grain loads with ballistic information for a 4" barrel. So you don't really know what is going to happen when you drop another inch off the barrel. However, if you try to keep the velocity up by going with the 100 grain bullet, you'll likely lose penetration, and you'll be knocking on the deck-hatch down into .380 territory.
So you could go with the +P 135 grain Critical Duty load, but that is formulated for duty weapons with longer barrels, so i don't know now it will perform in your 3.1" Shield barrel.
Corbon says, for their 115 grain +P DPX in a 4" barrel: 1250 fps/399 ft-lb
Hornady says, for the 135 grain +P Critical Duty in a duty length barrel: 1110 fps/369 ft-lb
Hornady says, for the 115 grain Critical Defense in a 4" barrel: 1140 fps/332 ft-lb
Hornady says, for the 100 grain Critical Defense in a 4" barrel: 1125 fps/281 ft-lb
Of those four loads, the Corbon produces the best results in a 4+" barrel. That's the one you've got to go with if you're going to cut an inch or more off your barrel length.
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