Thing is, damage is based on both the power level and the speed of the bullet. Which means a spas shot, which moves slow but kills at close range won't might not do nearly as well at long range because it loses speed much faster than, say, a barrett shot. So, if you want a
consistent 6 shot kill, set the speed really high and screw around with the damage values, because with a high enough speed level, it won't vary enough to make a difference.
Of course, you also have to worry about whether you hit them in the head/feet/body ect. So no, you can't have a gun that
always kills in 6 shots.
*EDIT* Oh, check out the fan apps section; if I remember correctly Avis made a program to help with damage calculating.
*FURTHER EDIT* It's
here, but I'm not sure whether it can help with modded weapons, or just the basics.