Staser

Staser weapons were the standard armament of the Chancellory Guard on Gallifrey. The Guards were normally equipped with staser pistols, although staser rifles were also occasionally used.

Usage
Stasers had both lethal and non-lethal settings, and fired an unspecified type of directed energy. They were relatively non-damaging to inorganic matter but caused massive tissue disruption to living creatures. (DW: The Deadly Assassin)

It would seem single staser shot could apparently kill a Time Lord outright and ensure that regeneration could not take place.

Stasers were ineffective against Sontaran armor. (DW: The Invasion of Time)

Staster pistols apparently come in different versions, those seen after the Sontaran invasion are significantly larger then those seen previously. It is possible that as the older variety were ineffective against Sontaran armour, a newer more powerful version was issued to the Chancellery Guard.

Instances of use

 * A staser wielded by Commander Maxil was used to stun The Doctor. (DW: Arc of Infinity)
 * The Un-named Castellan supposedly kept a small staser hidden on his person, and was shot dead by security personnel whilst using it to attempt an escape during Borusa's Death Zone gambit. (DW: The Five Doctors), however it is implied that he was murdered and the staser placed on the body.
 * A Staser sniper rifle was used in the assassination of the Lord President by Goth. (DW: The Deadly Assassin)

Behind the Scenes
The visual effect of a staser bolt varies, and has been observed as a white flash at the victim, a green lance, or a green-white beam that cascades over the victim's body.