BB,
I gotta agree with the rest of the guys here....
Sometimes minimum's below listed data can get a little hairy, for several of the reasons listed.
.223 minimum's that I have listed show velocities bottomed out at around 3000fps. with the 40gr. bullets.
.22 Hornet velocities max out at around 2800 with the same weight.
The two cartridges use totally different powders to achieve these velocities, which could get you into trouble there, as the other guys said.
The Hornet uses mostly pistol powders, i.e., 2400, 4227, H110, etc.
The .223 uses 4895, 3031, 4064 & H322 on it's minimum velocities.
To avoid "pass through" shots on fox, my preference would be to use a Hornady VMax bullet in either, your preferred choice of 40grs., or go a little heavier with a 50gr. bullet to reduce your velocity even more.
The VMax's, seemingly more so than other bullets, have a tendency to distrupt totally within most game, even at lesser velocity.
If you can find load data for the .223 that shows less velocity with the 40gr bullet, and it is of a reputable source, use it. If not, using a heavier bullet will bring the velocity down more, but might also promote a pass through shot due to it's lowered velocity.
If you do decide to go to a heavier bullet, try some of the "Hornet specific" bullets which are designed to expand, or detonate at those velocities. Make sure you get the .224 diameters, instead of the smaller .223's.
Thinner jackets and probably a little better overall anyway.
Kind of a balancing act I guess.....
Take care,
Bob