Question: Which type of weapon was the Provisional Irish Republican Army using in it's attack on Heathrow Airport

Context paragraphs:
Timeline of Real Irish Republican Army actions: This is a timeline of actions by the Real Irish Republican Army, also called the Real IRA, an Irish republican paramilitary group.  The group was formed in late 1997 by members of the Provisional Irish Republican Army who disagreed with that organisation's ceasefire.

Joe Cahill: Joe Cahill (Irish: Seosamh Ó Cathail ; 19 May 1920 – 23 July 2004) was a prominent figure in the Irish Republican movement in Northern Ireland and former chief of staff of the Provisional Irish Republican Army (IRA).  He joined a junior-republican movement, Na Fianna Eireann, in 1937 and the following year, joined the Irish Republican Army.  In 1969, Cahill was a key figure in the founding of the Provisional Irish Republican Army.  During his time in the Provisional IRA, Cahill helped import weapons and raise financial support.  He served as the chief of staff in 1972, but was arrested the following year when a ship importing weapons was intercepted.  After his release, he continued to serve on the IRA Army Council and lead all financial dealings for Sinn Féin.  In the 1990s, the IRA and Sinn Féin began to work on seeking peace.  Cahill served on the council that called a cessation on 21 July 1996.  Cahill attended several of the talks that finally led to the Good Friday Agreement on 10 April 1998.  Shortly after the agreement was made, Cahill resigned as treasurer of Sinn Féin.  To honour his service, he was made honorary Sinn Féin Vice-President for life.

Barrack buster: Barrack buster is the colloquial name given to several improvised mortars, developed in the 1990s by the engineering group of the Provisional Irish Republican Army (IRA).

Provisional Irish Republican Army: The Provisional Irish Republican Army (Provisional IRA) was an Irish republican paramilitary organization that sought to remove Northern Ireland from the United Kingdom and to bring about an independent socialist republic encompassing all of Ireland.  It was the biggest and most active republican paramilitary group during the Troubles.  It saw itself as the successor to the original IRA and called itself simply the Irish Republican Army (IRA), or "Óglaigh na hÉireann" in Irish, and was widely referred to as such by others.

Workers' Party of Ireland: The Workers' Party (Irish: "Páirtí na nOibrithe" ), originally known as Official Sinn Féin, is a Marxist–Leninist political party active throughout Ireland.  The party originated with a split between factions of Sinn Féin (which was founded in 1905) and the Irish Republican Army (IRA), which took place in 1969–70, early in the Troubles.  The faction that broke away became known as the Provisional Irish Republican Army and Provisional Sinn Féin.  The Officials' leaders at the time the Provisionals left were Cathal Goulding and Tomás Mac Giolla.

Rose Dugdale: Dr. Bridget Rose Dugdale (born 1941 in Honiton, Devon), better known as Rose Dugdale, is a former debutante who rebelled against her wealthy upbringing, becoming a volunteer in the militant Irish republican organisation, the Provisional Irish Republican Army (IRA).  As an IRA member, she took part in the theft of paintings worth IR£8 million and a bomb attack on a Royal Ulster Constabulary (RUC) station using a hijacked helicopter.

1994 Heathrow mortar attacks: The 1994 Heathrow mortar attacks were a series of homemade mortar bomb attacks targeted at Heathrow Airport carried out by the Provisional IRA.  Over a five-day period, Heathrow was targeted three times (9, 11, and 13 March) by the IRA, which fired 12 mortar rounds.  Heathrow was a symbolic target due to its importance to the United Kingdom's economy, and much disruption was caused when areas of the airport were closed over the period due to the IRA attacks.  The gravity of the incident was heightened by the fact that Queen Elizabeth II was being flown back to Heathrow by the RAF on 10 March.

Real Irish Republican Army: The Real Irish Republican Army or Real IRA (RIRA), is a dissident Irish republican paramilitary organisation which aims to bring about a united Ireland.  It formed in 1997 following a split in the Provisional IRA by dissident members, who rejected the IRA's ceasefire that year.  Like the Provisional IRA before it, the RIRA sees itself as the only rightful successor to the original Irish Republican Army and styles itself as "the Real Irish Republican Army" in English or "Óglaigh na hÉireann" in Irish.  It is an illegal organisation in the Republic of Ireland and designated as proscribed terrorist organisation in the United Kingdom and the United States.

John Kelly (Sinn Féin politician): John Kelly (5 April 1936 – 6 September 2007) was an Irish republican politician in Northern Ireland.  He joined the Irish Republican Army in the 1950s, and was a founder member and a leader of the Provisional Irish Republican Army in the early 1970s.

Timeline of Continuity IRA actions: This is a chronology of activities by the Continuity Irish Republican Army (CIRA), an Irish republican paramilitary group.  The group started operations in 1994, after the Provisional Irish Republican Army began a ceasefire.

Supporting paragraphs: 1994 Heathrow mortar attacks Barrack buster
Answer: fired 12 mortar rounds