Code to be Added or Changed
g_weapon.c & g_missile.c
As we know each weapon does a differing amount of damage, and if you reduce a players health to -40 they will explodes in a shower of gibs.
Single Weapons: So all we have to do is change the amount of damage done (see the weapon damage tutorial) by the weapon to something like 1000 then its always going to gib the opponent.
All Weapons: However, if you want every weapon to be instant gib then the easier way is to just change the gib value from if health -40 to something like if health -5, resulting in a far greater change of gibbing. You'll find that value at the very top of bg_public.h in the list of defines:
#define GIB_HEALTH -40
Change it to #define GIB_HEALTH -5