The IRA's Mark 15 "barrack-buster" mortar first appeared at the end of the 1980s. The mortar tube is about four feet long and is made from a gas cylinder cut at either end by an oxyacetylene torch.
The tube is welded to a base plate through which a fuse is passed to a propellant charge.
The fuse is then attached to a timer power unit (TPU), which can be set for multiple or single launching. The multiple-launch timer allows enough time between each launch for the firing platform to settle so the following mortar is not thrown off target.
This multiple-launch system can be welded in a frame made from angle iron, which is attached to the back of a tractor or into the back of a van. The roof of the van is cut off and replaced by cloth.
Early IRA mortars were unreliable, and several IRA members killed themselves when they misfired. A number of British army ordnance officers were also killed or injured trying to defuse them.
By the mid-1980s the IRA had perfected the weapon. The most devastating attack by mortar was on Newry RUC station on February 24th, 1985, in which nine officers were killed when the missile made a direct hit on a temporary building used as a canteen.
The British army and RUC embarked on a campaign to improve defences at their barracks. Almost every police station in the North was rebuilt with mass reinforced concrete and steel meshing.
One of the policing reforms currently being proposed is for the removal of these defences to make stations more accessible to the public. This may stop if the "Real IRA", which has taken over the use of the IRA's Mark 15 mortar, succeeds in an attack on a security base.