Khan's residence Banigala is in Islamabad suburbs where he lived before shifting to the official residence.
The bullets seized from the plot is located about half-a-kilometre away from his private residence where he often visits.
A senior officer of a foreign mission noticed the ammunition and informed the police, Dawn news reported. A patrolling team cordoned off the area.
Teams from different police wings, including Counter-Terrorism Force, Crime Investigation Agency and Bomb Disposal Squad seized 18 bullets of the anti-craft gun. The bullets, seemed to be old, were 30mm in length and de-coloured, a police officer said.
Security has been stepped-up in the area.
Police suspect that someone had brought the ammunition to the plot during the last a couple of days and tried to hide them in garbage. The ammunition were shifted to a safe place and would be sent to a lab for forensic analysis, said the officer. So far, nobody claimed responsibility for placing the ammunition near Khan's residence.