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Fortifications of Operation Banner

Between 1969 and 2007, both the British Army and civilian population were under attack. It was a dynamic conflict, both in terms of geography and threat, during which there would not necessarily be a winner, just a lesser loser. And there were three sides; republicans, loyalists and the security forces, which incorporated the RUC/PSNI, British Army and intelligence services. It could probably be agreed that the side that incurred not the greatest loss of life, but the greatest volume of attack from the other two was the British Army. They had predictable routines, fixed bases, and generally very overt vehicles. These three elements became the focus of hatred from both sides, but in particular the threat from republican paramilitaries was greatest, and it was this threat that prompted the greatest evolution of protective engineering.

There seems to be little published material on the development of fortified observation posts or sangars, so in this article, I want to look at some of the engineering solutions that were put in place to protect the security forces, in particular fortifications that grew up in sometimes the most unsuspecting places, creating a guide to identification, and where possible looking at their performance. You may also find my article “How thick is thick enough?” assessing the protection offered by military field fortifications.


As far back as 1950, the Ministry of Home Affairs was asked to arrange for additional protection at RUC stations.1PRONI HA/32/2/69A Royal Ulster Constabulary Security Measures, page 29 At the 74 stations identified as being vulnerable to attack, it was requested that they fit steel shutters and wired glass on the ground floor windows in the station, with the exception of small windows in ancillary rooms such as stores and larders. At this stage in the campaign the threat was from a few armed assailants, most likely intent on stealing the weapons inside the station as opposed to murdering the occupants. Arrangements were also made to strengthen the inside of the station doors in order to prevent them being “blown in by explosives.2PRONI HA/32/2/69A Royal Ulster Constabulary Security Measures, page 27” In what seems a rather simple solution, two strong steel wires were fitted across the inside of the front and back doors, and if they did not hold the door during an explosion they would certainly “impede the raiders in making an immediate entry.” In a memo dated 1956, this work was still not complete across all stations.

For six years between December 1956 and February 1962, the Irish Republican Army (IRA) waged war in what was to be known as the Border Campaign. This new war aimed to attack military and infrastructural targets within Northern Ireland using a series of guerrilla units. It was raids earlier in the decade between 1951 and 1954 on military and police establishments that armed the newly determined IRA, and the security forces in Northern Ireland started to respond by increasing the protection and security at key points across the country.

A sandbagged protection post outside a RUC station. I have not been able to establish the source or date of this image.

1956 also saw the start of the construction of dedicated emplacements at RUC stations. In a document dated 31 December 1956 it was reported that “arrangements have been made for the erection of protection posts outside 103 RUC stations and establishments. The posts will be constructed with sandbags, barbed wire and other necessary materials.” This marks a step change from passive to active protection, a change that would endure for the next 50 years, and beyond.

At this stage it is important to note that all protective work was completed through civil departments, via requests from the RUC. It was not until 27 April 1969 when the first squadron334 Field Squadron of Royal Engineers deployed to Northern Ireland to “construct watch towers, sentry posts and wire defences against the threat of attack on public utilities and government installations.4The History of the Corps of Royal Engineers, Volume XI, Chapter IX” Due to the risks involved, the next phase of fortification was constructed almost solely by the Royal Engineers, however in 1969 the general opinion form official sources seems have been that the current civil unrest would be short lived. History has told us otherwise.

A crude beginning

It was a rough start for troops deployed to NI in 1969; they had been trained in conventional warfare and equipped with war fighting equipment, not for what should have been community relations in urban areas. It is not for me to speculate on the development of the civil disturbances of 1969 into the shooting war of the 1970s, but from the tactics employed by the army chiefs in form of protection on the ground, there were definitely three distinct phases that reflect the ethos of the security forces and the evolving threat of the well armed and predominantly republican terrorist:

  1. Crude field fortifications between 1969 and 1972
  2. Semi-permanent bullet proof structures of the 1970s and 1980s
  3. Permanent bullet and blast proof structures of the 1990s and beyond

The image below appears to be the earliest I can find of one of these crude field fortifications, more akin to a trench position constructed without digging in.

What appears to be an early sandbag position. This nature of position is very much a field fortification and would not offer much protection from protesting crowds or the Northern Irish weather. I think the location is North Belfast, perhaps along the Shore Road, with the shipyard in the background. The presence of a Pig Armoured Personnel Carrier (APC) in the background dates this image to post-April 1971. This image is from another history blog but has no associated information as to their source.

A good example of the enduring nature of the initial constructions was at Creggan Estate Camp in Londonderry where in the summer of 1972 a base camp was constructed over looking the Creggan Estate 5Royal Engineers Archive, Chatham. Document number unknown.. This formidable fortification had a defensive sandbag wall over 100m long and 3.65m high consisting of more than 40,000 sandbags and 1,100 tons of sand dredged from Lough Foyle. In the damp conditions of the Northern Irish air, the hessian bags started to decay, the wall started to crumble. Remedial work was carried out in March 1975 and in 12 days engineers from 12 Field Squadron, Royal Engineers reinforced the perimeter in chicken wire and sprayed it with a commercial concrete mixture called Gunnite.

An Observation Post at Mayo Street in 1970. The image is from a website that is now offline.

As well as providing cover, there was an early requirement to provide observation posts. Without any standard of design and with limited materials available through the Royal Engineers, crude scaffold and corrugated iron OPs appeared. Their elevated position enabled soldiers to cover large areas, but with only a simple roof there was little in the way of weather protection and there was no ballistic protection provided. The OP in the image (right) would form the basis for the next phase of construction.

In the coming months observation posts would become armoured and observers would become snipers. I am sure at the time orders would have been akin to “observe, unless you’re shot at, then shoot back.” But when your adversary looks like, talks like and dresses like the rest fo the civilian population; the problem compounds.

Observation Post Design 1972

It’s important to note at this stage that 1972 was the single most deadly year of the operation, the Security Forces (police, Army, UDR) suffered 146 deaths, there were 223 civilians killed and 98 terrorists form both sides (95 republican, 3 loyalist)6The History of the Corps of Royal Engineers, Volume XI, 1960 – 1980. The main form of attack was shooting, rising steadily from 399 recorded incidents in March 1972 to the peak of 2,718 incidents in July7An Analysis of Military Operations in Northern Ireland, Army Code 71842. Crown Copyright. 1972 was also the year of Bloody Sunday and tensions were high.

Two designs of OP emerged from the Royal Engineers in 1972, both designs available from the Royal Engineers archive in Chatham. A lightweight standard design of OP, and an armoured variant. Both being timber framed, and both could be constructed at ground level, on top of a flat building roof or elevated on a timber of scaffold frame. Neither design could be considered permanent and would offer little protection to anything other than small arms or rifle fire; and this very much reflected the threat at the time.

Standard OP

The standard OP was no more than a timber frame and roofed OP to give soldiers shelter while on duty. The only ballistic protection came from 600 sandbags that were arranged in two rows around the base of the post. This would have given them protection from small arms fire, however there was no protection over the observation window, not from the weather nor from fire. NB: It takes 75cm of compacted sand in bags to provide protection from bullets.

An image of a 1972 standard sangar at Crossmaglen constructed for the Argyll and Sutherland Highlanders © RHG Argylls Association
Sheet corrugated aluminium (10′ x 2’2″)5
Sheet corrugated mild steel (6′ x 2’2″)30
Timber softwood
Plywood sheet (8′ x4′ x 1/4″)1
Sand12 tons
Sand bags600
Drive screws25 dozen
Washer curved diamond25 dozen
Nails wire (6″ and 4″)40 lbs
Bolts (with nuts and washers)12
Stores list for the 1972 standard OP design. Contained in Appendix 6, Annex E to the NI Tour Report from 34 Field Squadron Royal Engineers

Armoured OP

From the outside, the armoured OP did not vary much in design from the standard design, however the use of a 10mm armoured plate skin on the outside, with a cement and sand mix fill between the steel exterior and plywood interior, this was starting to form the basis of the sangars we would become so familiar with. The roof was constructed of a mild steel sheet, covered with sand bags and clad in a corrugated iron sheet for weatherproofing.

The construction of the armoured OP in detail showing the armoured external skin, the plywood interior and a fill of sand and cement. The skins were bolted together with the external steel plate being welded at the seams.
Armour plate (10x1470x1845mm)6
Mild steel plate (6x910x1820mm)2
Corrugated iron sheets (3m long)8
Bolts, nuts and washers80
Timber sawn100 m
Timber planed8m
Nails wire22 lbs
Screws and diamond washers40
Sand1.2 cubic meters
Cement4 bags
Stores list for the 1972 armoured OP design. From drawing number 34/NI/72/1 from 34 Field Squadron Royal Engineers

Makralon Sangar

Makrolon™ is a branded polycarbonate that offered basic ballistic protection, in particular against low velocity small arms fire; such as hand guns and sub-machine guns. It was extensively used in the early 1970s as a lightweight armour kit for land rovers and other soft-skinned vehicles. There is also reference to Makrolon™ Observation Posts in a 1972 report on the activities of 8 Field Squadron Royal Engineers. I have not been able to identify any images of this variety but the report specifies this variety was installed in the New Lodge, Finiston School, Louisa Street, Andersonstown and Glasmullan Fort, while an installation of this type was cancelled at Girdwood Park.


In a memo sent form the Security Co-ordinator on 7 November 1979 for Belfast City Centre, a joint RUC/Army sangar design was proposed that would be a permanent feature of the city centre. This work fell under Op MINDER, the strengthening of security measures in Belfast. Following a standard design, each one would vary slightly depending on the requirements of a particular area. They would be occupied by four men, with a raised observation platform covering three directions, and have basic living accommodation downstairs. The sangars will “provide protection for the crew against blast bombs and sniper bullets and they will be able to control the lowering of the barriers from inside the sangars where the soldiers will be able to cover the barrier by observation and, if necessary, fire.

An artists impression of the sangar design proposed for Belfast City Centre in 1979. Accommodation for four crew, or half an army Section.

Of this design, eleven sangars were proposed, many of which would require a requisition order for the land.

There is no advantage in seeking to draw attention to these constructions which is not more outweighed by the disadvantages of making the Catholic population as such nervous of our intentions and of disclosing out hand to the Provisionals.

A statement in a secret memo from the Security Coordinator on the expansion of security measures around Belfast
Crumlin Road OP – While the location of this permanent Observation Post does not feature in the 1972 proposal, it appears to be the only image I have yet found of this type of self-contained OP. It was located on the four-way junction of Crumlin Road / Woodvale Road / Twaddel Avenue. It is surrounded by rocket cage and reinforced concrete blocks. Image from The History of the Corps of Royal Engineers, Volume XI, Chapter IX, no other credit available.

Transportable Sangars

SCS Composite Sangar

The steel-concrete-steel (SCS) composite sangar was a clear development of the standard armoured design of 1972. Two 10mm steel sheets sandwiched a 100mm core of concrete could protect the occupants against a 0.5″ bullet fired at close range. It was prefabricated elsewhere which meant in high threat areas it required a much shorter time to install or remove, as little as 30 minutes.

An SCS sangar being installed (or possibly removed?) at Crumlin Road in Belfast. I haven’t been able to place this image using what I think is a roundabout in the back left and the church back right of the image. The original source and date are unknown.
An SCS at Crossmaglen security force base. This SCS has rocket screens added. Image from Pinterest, user Bones Balkham-Smith.

I have not been able to establish the service dates for the SCS sangar but between the mid-1970s and 1988 seems a sensible calculation. In the Royal Engineer archives at Chatham I came across a note in a unit report about the construction of the elevated sangar at Crossmaglen; I can presume this refers to the SCS sangar in the image above. It was constructed by engineers from 53 Field Squadron in 3 hours. The sangar was made in prefabricated sections that were flown to the site by helicopter.

MAT Sangar

The Mobile Adjustable Threat (MAT) sangar was the first standard pattern transportable sangar deployed in NI and came into service in 1988. To ease with lifting the sangar into and out of locations quickly, it was fitted with steel lifting eyes which can be seen in the image below.

A soldier was killed inside a MAT sangar in May 1994. They were designed for ballistic protection, not for blast protection. This event quickly saw the MAT sangar go out of service. Image is a screenshot from an © Associated Press news story w066815

Additional protection was added to the MAT in 1991. The additional steel plate which was bolted onto the outside, below the windows, could increase the protection to 0.5″ shots. You can see the additional plates in the image above. Unfortunately it was the windows that made it weak to blast, and while the armour was most likely un-penetrated by the blast, the windows blew in and killed the occupant. Future sangar variants had much strengthened window mountings.

I think this is a modified MAT sangar, smaller than the MARS below but with the blast proof window boxes. This variant appears to be common around NI from the 1990s onwards. Image source and owner unknown
The MAT sangar can also be seen in more permanent structures. Here the sangar has been clad in stone, perhaps in an attempt to normalise its presence. The rocket screens and barrack fence detract slightly from this attempt, but it could certainly be seen as a move towards permanence © Castle of Ulster, Jonathan Olley

Multi-Armoured Role Sangar

The Multi-Armoured Role Sangar (MARS) became the new standard for armoured Observation Post in NI. Entering service in 1995, I am unaware if there was a MAT replacement scheme, or all new sangars constructed became the MARS pattern. With the improvements to MAT sangars it seems likely that upgrades took place as opposed to more costly replacements. As standard, the MARS single thickness 50mm laminated glass was designed to protect against the most common threat; 7.62mm Armoured Piercing (AP) (for information, the AK variant assault rifles fired a 7.62x39mm projectile and had armoured piercing ammunition available).

Between August 1992 to December 1993 the ‘South Armagh Sniper’ known locally as ‘Goldfinger’ killed 6 soldiers and 3 RUC constables using a .50″ calibre sniper rifle.

As the IRA waged a sniper war with their 0.50″ rifle, the MARS glass could be upgraded to double thickness and the body was already capable of stopping a 0.50″ round.

MARS overlook the Maze Prison © Frontline Ulster

After decommissioning from Northern Ireland in around 2001, the MARS were sent to Afghanistan, where the threat was similar to that faced in Northern Ireland. The hot climate however proved to be incompatible with the steel and concrete boxes, so their use was limited. It also appears that the windows appear to have had an upgrade from the NI deployed sangar.


In the mid-1980s there were two operations to construct the border permanent observation posts; Op CONDOR to build the hilltop OPs in the Forkhill area, known as the High Romeos, and Op ENTIRETY to cover the approaches to Crossmaglen and the low lying Golf towers. The initial towers had been up for nearly a decade and a half and most supported on standard scaffold towers. The introduction into service of the Cuplock scaffold system in 1998 could speed up construction times and support a sangar and rocket screen anywhere between 3m and 13.5m high. A variation of the MARS was designed specifically to be air-portable for installation at these border watchtowers.

MARS 2000

With much of the infrastructure earmarked for demolition in 2001 following the latest IRA ceasefire, the new construction or upgrading of fortifications was low on the priorities. However, one final design of transportable sangar was introduced as an upgrade to the Multi-Armoured Role Sangar. The MARS 2000 saw increased blast performance, in the main due to the introduction of single sheet steel construction (fewer welds and weaknesses) as well as sloped armour and windows. The MARS 2000 upgrade was essentially a retrofit armour package over a standard MARS; you can almost see the MARS above and below the sloped armour. The sangar upgrades were drawn up by the Design Cell Royal Engineers and manufactured locally.

Possibly the only example remaining, a MARS 2000 sangar outside Kinnegar Logistics Base on Belfast Lough. Image copyright the originator (via private email)

Concrete Sangars

As both the threat continued to develop, so did the expectations of longevity and normalisation associated with defensive structures. There was no longer need to hastily air lift sangars into remote locations, but there was an increased need to maintain the security around the dwindling number of security force bases, police stations and courthouses. Increasingly the car bomb was being deployed against permanent installations, while mortar and gun attacks had declined considerably. The prefabricated concrete sangar was used in conjunction with new commercial protective solutions such as blast resistant materials and walls. While it is possible to obtain information about the commercial products, plans and information about sangars which are likely still installed around secure bases will not be available in the public domain for decades to come.

By 1983, the majority of fortifications are reported to be constructed with either high density concrete blocks, or Christchurch blocks, with sangars utilising reinforced concrete slabs in their construction8From a report written for HQNI on the integrity of concrete following attack by 0.50″ bullets.

Permanent Vehicle Checkpoints (PVCP)

The PVCP was a key tool in preventing the free movement and smuggling of weapons, ammunition and people across the border between north and south. They were often joint RUC / Army operations set up on main trunk routes in border regions. They were also isolated and vulnerable to attack. As permanent fortifications, they could be constructed from more durable materials. One sangar design that appears to have been used frequently in PVCPs was the single aperture sangar.

It is difficult to establish the timeline of construction, but there also appear some three aperture sangars at border PVCPs. Unfortunately there are not enough dated images for me to create a definitive categorisation, but it appears that sangars of this type emerged in the 1980s and endured until the demolition of all PVCPs in the mid to late 1990s (table below)

A double level, triple aperture sangar in the foreground of this image of a PVCP somewhere in NI. The date and credit for the image are unknown.
YearClosure / Demolition
1995Kilturk PVCP, Fermanagh
Clady PVCP, Tyrone
Buncrana PVCP, Londonderry
1997R16 PVCP, Newry
1998R16 roadside sangar, Newry
Bessbrook PVCP (x3), Armagh
1999Aughnacloy PVCP, Fermanagh
The Hump PVCP, Strabane
Muff PVCP, Londonderry
Dates of closure or demolition of the border Counties PVCPs. Along with the closure of PVCPs, many Patrol Bases also closed.

Civil variations

On researching the typology of modern sangars, it has been very difficult to establish a distinct single pattern that has been utilised throughout the country. This may in part be to the slowing of construction and the legacy of some sangars being in place and not removed for decades. In other areas it is clear that modern materials have been used to fit into the urban environment in which they are being installed. In all cases, they appear to be concrete in construction and while they are still recognisable sangars, they share little with the crude and transportable cousins of the early Troubles period.

Mortar-proof buildings

As the threat developed from just being small arms to high-velocity armour piercing ammunition, so did the sangar construction. When the IRA developed and started using improvised mortars to deadly effect. In the rush to accommodate troops across the country, a large amount of prefabricated accommodation was used, which offered little to no protection from mortar attack. Tests were conducted to assess the best protection against the new threat, the result of which was to increase the thickness of the accommodation roof by using additional layers of chipboard. However as these mortars increased in size, this method was inappropriate.

An early design of improvised mortar that the IRA were launching against soft targets such as prefabricated accommodation blocks used inside the hastily constructed camps. Image from a 1973 report from 62 CRE, National Archives reference WO227/121

The main method of protection of a building was to initiate or detonate a mortar “not closer than 2 feet to the building fabric9WO 227/121 in the National Archives, dated 12 December 1973.” During development, a number of ideas were mooted, which although were never fully implemented, the principles were retained:

  • Waterproofed Kapok mattresses
  • Multiple layers of Stranit
  • Slabcon
  • Sandbags
  • Foam concrete
  • Glass reinforced concrete

The time and cost of providing measures to reinforce or protect the fabric of the vast number of accommodation buildings in use by 1973 was deemed too great, but immediate protection to could be provided for high priority buildings using a thick layered chain-link screen fixed at a standoff of greater than 2 to 3 feet from the building. Using this method, the bomb would be trapped, deflected or initiate early.

By 1977 the Royal Engineers were designing a mortar-proof 72-man accommodation block to be constructed inside the security force compound, adjacent to Crossmaglen RUC Station. The design10From documents in the Royal Engineers archive at Chatham was “a bonded blockwork building, buttressed at intervals, with glass reinforced plaster inside and out. The roof to be made of reinforced concrete with a 9 inch cover of sand.” Remarkably I have obtained a copy of the plans for this building from the National Archives, and have managed to find a photograph from 1989 of what appears to be this very same building following an IRA mortar attack.

The mortar-proof accommodation block at Crossmaglen following at attack in 1989

Additional protection

For many security force bases, the sangar was the only fortified building. It would have been a prime target for attacks, and many sangars bore the brunt of both terrorist attention and civil disobedience including paint and petrol bomb attack. They were rarely deployed alone and other protective measures were put in place.

Sniper screens

A simple corrugated iron (CGI) screen was used around most security force bases to enable soldiers to move around without the fear of being targeted by sniper. This invisibility was removed when they left camp, and this departure, when taken on foot, was often brief as soldiers ran to the nearest point of hard cover!

Sniper screens at a Permanent VCP in Fermanagh

Mortar nets

Wire nets shrouded many security force and police buildings and compounds throughout the Troubles. They were intended to catch, divert or prematurely initiate mortars launched against the targets. Typically deployed against smaller targets, such as covering a sangar or in the example below protecting an alleyway. For larger areas a bomb catch net was used.

Mortar nets outside Donegall Pass police station

Bomb catch nets

Where it wasn’t possible to erect a net o ver a large area, tall bomb nets were constructed designed to deflect bombs away from the compound. This form of net was often added to the top of sniper screens or blast walls.

Bomb catch nets outside a Belfast police station in the 1980s from the National Library of Ireland Photographic Archive

Rocket screens

When the RPG became a threat to security forces, the armoured sangar that was once impenetrable to anything up to .50″ high velocity rounds was suddenly at risk. The solution was the addition of rocket screens. They came in varying forms, but the method of operation was the same; by initiating the armoured piercing warhead early, and at a considerable standoff from the fabric of the sangar, then the armour penetrating capability is reduced.

Anti-vehicle blocks

Concrete blocks have been used to stop or delay vehicles since the Second World War. During Op Banner a number of designs were devised that could be easily transported and set up to block or direct traffic flow, and prevent vehicles from getting close to security force bases or checkpoints.

A selection of concrete obstacles photographed in 2007 on the edge of the Long Kesh site. Blocks like this would have been used to prevent vehicular access close to barracks and checkpoints, but also to restrict access in built up areas such as Belfast city centre © Frontline Ulster

Blast panels

A late addition in the range of protective measures introduced during Op Banner, the reinforced concrete blast wall mitigated against the growing threat of large car bombs. Simple reinforced concrete panels soon became obsolete due to the release of spall (concrete fragmentation) as the blast wave propagated through the concrete. This developed through the addition of a steel spall plate on the inside.

The entrance sangar at Girdwood Barracks in Belfast appears to be a modified SCS sangar. It has been clad with a corrugated metal sheet, but it is unknown if any additional ballistic protection has been added. It also has a steel rocket screen fitted; this increased the protection level against RPGs only. The compound was surrounded in reinforced concrete blast panels © Frontline Ulster

Three commercial products were evaluated and introduced during the final decades of the Troubles were the Redline panel incorporating the steel spall plate on the rear that was cast in place using hollow steel hoops to increase cohesion between the steel and concrete. The Redline 2 incorporated a panel front and rear. Another product called Bi-Steel, similar to the Redline 2 could be produced mechanically and much more quickly. Niccola resistant panels were also documented as being used to surround the base of elevated watchtowers to protect soldiers as they ascend and descend the towers. I am yet to find definitive pictorial examples of these blast walls that I can positively identify.

Protected plant

It would be remiss not to mention the range of fortified construction equipment that came from the construction of observation posts and patrol bases. As violence escalated, and as the Royal Engineers took on a greater number of tasks, often being the first soldiers setting foot on a new site, it was realised that protecting them from small arms fire was essential. In 1972 a requirement was issued to the Military Vehicle and Engineering Establishment (MVEE) at Christchurch in Surrey to equip 48 wheeled plant with armour to withstand high velocity and armour piercing rounds. This was aptly known as Op TURTLEBACK.


I started out writing this piece with the impression there was only a small amount of variation in sangar design throughout the Troubles. It will take a considerable amount more research to create the academic guide that I strive for, and perhaps I will be afforded that opportunity in the future. For now, I hope you have found the results of my research enlightening and I hope you will look twice next time you see an elusive armoured sangar in the future.


I couldn’t help but to include a sample of this stock image from Alamy. In around 2007/2008 all of the sangars that had recently been removed from watchtowers, barracks and police stations were brought to Kinnegar Logistics Base, near Holywood. At the time I passed them but didn’t stop and ask to take a photograph, and that’s something I regret! This photograph, taken by photographer Stephen Barnes has captured the great evolution of the transportable armoured sangar in Northern Ireland, including an early concrete example in the background.

Decommissioned sangars await disposal at Kinnegar in 2007. Sample image from Alamy © Stephen Barnes

  • 1
    PRONI HA/32/2/69A Royal Ulster Constabulary Security Measures, page 29
  • 2
    PRONI HA/32/2/69A Royal Ulster Constabulary Security Measures, page 27
  • 3
    34 Field Squadron
  • 4
    The History of the Corps of Royal Engineers, Volume XI, Chapter IX
  • 5
    Royal Engineers Archive, Chatham. Document number unknown.
  • 6
    The History of the Corps of Royal Engineers, Volume XI, 1960 – 1980
  • 7
    An Analysis of Military Operations in Northern Ireland, Army Code 71842. Crown Copyright
  • 8
    From a report written for HQNI on the integrity of concrete following attack by 0.50″ bullets
  • 9
    WO 227/121 in the National Archives, dated 12 December 1973
  • 10
    From documents in the Royal Engineers archive at Chatham