From QuakeWorld Wiki
Instagib is a known mod for QuakeWorld first introduced in KTPro. The instagib gametype started with quake 2, using the railgun. It has been used in several other games, becoming popular with Unreal Tournament, where it was combined with CTF and DM.
The handling of the weapon could be compared with the railgun from games such as Quake 3. The difference being that every hit is guaranteed to be lethal, from any range. Ammo is unlimited. After a few years of stagnancy in the development, Deurk from KTX continued the development of the mod in October 2007 and first implemented it in KTX v.???. Reload times are the ones from the Shotgun or the Super Shotgun. Instagib can be played on any map.
 Instagib in KTX
In KTX, typing "instagib" into the console will not only allow you to toggle the mod on/off, but also to modify the speed of the weapon's reload time. The choices available concerning the reload time are "SG" (Shotgun) and "SSG" (Super Shotgun). To make any of the above changes, the server is required to be set to dmm4 first.
 Instagib in KTPro
In the original (old) KTPro instagib mod, instagib could be toggled on/off by typing "instagib" into the console. If the server is set to dmm8, the player will be able to use the Lightning Gun as a sort of railgun, which will shoot a thin, purple-coloured lightning straight at the targeted destination. Every kill will, regardless of the weapon used, result in 1 frag.
The weapons available in the game are limited to the Axe and a new weapon called the "Coilgun". Killing an enemy with your Axe will award you with 2 frags, compared to 1 frag with the coilgun. The coilgun will behave similarly to a railgun, the difference being that it shoots a high-velocity, infinite coil rather than a lightning. Additionally to the above mentioned methods of earning frags, it is also possible to stomp your opponent to death. This will be awarded with 4 frags.
Due to every shot being lethal, staying alive in instagib is not an easy task. Thus the KTX developers thought of a method to award not dying. Every picked up backpack will increase your health +10. Picking up 5 of these backpacks without dying in between, thus having 300 (or more) health, will make you invisible for 30 seconds, making it easier to kill your enemy using the axe and scoring double the points for a kill.
Three different weapon models are available for instagib play on KTX servers. The player can either us the Shotgun and Super Shotgun, depending on the reload speed selected, or the brand new Coilgun model by Orion. Instagib in KTX also has increased gibs for extra viewing pleasure.
 Other changes
The weapon used for instagib in KTX will have a "kickback" similar to the Rocket Launcher. It is thus possible to make "coiljumps", which are basically the same as rocket jumps but without an explosion being caused. Additionally to this, KTX will measure the height of frags made in midair. The results and statistics will be printed into the console at the end of the game.