You are talking about velocity, but the point is that the energy is going to be the same. Unless the weapon is so heavy to increase the total mass of shooter and weapon to a significantly higher mass than the target, it's not going to make much difference.
Even if it is possible to devise a weapon that will knock down the BG but not the shooter, it would have to be a cumbersome weapon (very heavy to absorb enough recoil) that has an incredibly powerful cartridge. In other words, it's not practical to have something capable of this.
Oh, and at the point where you have that powerful of a weapon, it's not going to be easily stopped by armor.
You are already failing the test. I didn't venture a type of caliber or gun, but you did (service pistol).
I'm fully aware that a common service pistol won't do this.
It is possible to devise a weapon that will, however.
Even if it is possible to devise a weapon that will knock down the BG but not the shooter, it would have to be a cumbersome weapon (very heavy to absorb enough recoil) that has an incredibly powerful cartridge. In other words, it's not practical to have something capable of this.
Oh, and at the point where you have that powerful of a weapon, it's not going to be easily stopped by armor.