You'll need to tweak things in the arena, but you can make it happen with extreme SHOOT_FORCE and SHOOT_MAXVEL values.
In the 2033 mod I used to be making, there was a sniper rifle that did roughly what you're describing.
The gun had both variables set to 1mil - it routinely threw adult humans back 1-2 spaces with body/head shots, and easily removed hands/feet. Ammo was SIZE:60 and EDGE:20:120K.
That said, another one had both vars set to 60K, and even that would occasionally knock back particularly weedy people. Ammo was SIZE:45 and EDGE:20:50K.
Kind of a hack job, but it produced satisfying results. Would be even easier for you, I imagine, since in a Western setting you don't need to worry about body armour and penetration.
Making kevlar armour that saw no effect from weak rounds, broken bones from others, and full penetration from powerful ones took days.
BtW, if you're gonna put in guns at all, I would actually recommend playing with the tissues above all else. I got good results from MAJOR_ARTERIES in the brain, (throat), heart, and spleen, as well as lowering the pain value of bone/muscle and raising it for organs.
Wound up with a much more brutal and fast-paced combat system, in which a shot to the head or torso stood a good chance of killing or crippling. However, people could fight through multiple limb wounds or no-organ body shots.
The randomness of this system, I found, helps preserve the fast-paced feel of a gunfight. It took skilled marksmen about 10-15 rounds on average to kill a humanoid target, which while not perfect, is a step up from the immersion-breaking dozens of arrows that people routinely take in Vanilla.
Tried putting MAJOR_ARTERIES in the Upper Legs, too, but then nearly every hit to the thigh was severing a femoral artery. Morbidly funny, in a way, but stupid.
Aaaanyway, that kind of got rambly. Sorry. Hope some of it helps.