Another factor is bullet construction:
Jacket material
Jacket thickness
Bonded or unbonded
Jacket design (tapered vs. reverse tapered.)
A 9mm bullet with a thin non bonded jacket may penetrate ok at 9mm velocities that do not cause the jacket to fail or separate. However, if you push that same bullet a little faster the bullet may come apart in less distance, leaving smaller fragments which do not penetrate as far. Even bonded bullets may penetrate less in ordnance gelletin or actual tissue just because the bullet expands more rapidly and increases its drag coefficient earlier. Since most .357 Sig loads just use existing 9mm bullets that were designed to work in a velocity range that is up to 20% lower, there can be higher jacket failure rates, and higher expansion rates than when operating in the designed velocity envelope. If manufacturers built projectiles specifically for .357 Sig, it could be a bigger penetrator, at least with HP ammo.
Some folks may even prefer having less penetration, and more violent expansion that the 9mm bullets offer.
Just food for thought.
Jacket material
Jacket thickness
Bonded or unbonded
Jacket design (tapered vs. reverse tapered.)
A 9mm bullet with a thin non bonded jacket may penetrate ok at 9mm velocities that do not cause the jacket to fail or separate. However, if you push that same bullet a little faster the bullet may come apart in less distance, leaving smaller fragments which do not penetrate as far. Even bonded bullets may penetrate less in ordnance gelletin or actual tissue just because the bullet expands more rapidly and increases its drag coefficient earlier. Since most .357 Sig loads just use existing 9mm bullets that were designed to work in a velocity range that is up to 20% lower, there can be higher jacket failure rates, and higher expansion rates than when operating in the designed velocity envelope. If manufacturers built projectiles specifically for .357 Sig, it could be a bigger penetrator, at least with HP ammo.
Some folks may even prefer having less penetration, and more violent expansion that the 9mm bullets offer.
Just food for thought.