I've always had the opinion that people who commit/convicted of heinous crimes, should have heinous deaths.
I also would like to present to you my (I believe to be original) execution slide... construct a near vertical water slide , 75 feet ought to do it and add a nice launch ramp to the bottom of this slide. At the base or start of the launch ramp, make that bit of slide out of cheese grater blades, not much is needed, say 10 feet or so.
Think Water Country's Geronimo but with a spicy launch ramp.
Locate this slide out in the ocean about a mile off the Farallon Islands, a well known rock outcrop 30 miles offshore of California... LOTS of Great White sharks.
You ride the slide blindfolded and naked, whatever side you slide on is entirely up to you.
I believe this would be a proper deterrent and also satisfying to the victims surviving relatives.
But... if we have to use a fired projectile.
The runner up to the slide is gently place a 16" Mark 7 shell on the inmates chest while he/she is laying flat on the floor.
The beauty is, this one round can be used over and over... very cost efficient.