Building on its experience in the production of modernisation packages for the 155 mm M114 towed howitzer, fully covered in a separate entry, RDM Technology has developed an upgrade package forthe widely deployed US 105 mm M101 series of towed howitzer originally developed over 60 years ago.

When upgraded, the M101 can fire more modern ammunition with enhanced range and lethality. The modernisation is accomplished by using a maximum of existing gun parts and only adopting new components where required.

In late 1990, an upgraded M101/33 howitzer was demonstrated to the Canadian Armed Forces in Canada.

By late 1993, three prototypes of the upgraded RDM Technology 105 mm M101/33 howitzer had been completed. The third weapon underwent extensive trials in Indonesia, which were completed late in 1993.

In 1992, RDM Technology and Royal Ordnance of the UK reached an agreement whereby the two companies will co-operate in marketing their 105 mm artillery systems.

In March 1994, RDM Technology was awarded a contract by Canada to upgrade 96 M101 (plus an option on another batch of 36 which has yet to be exercised) into the M101/33 calibre configuration for the Canadian Armed Forces with the complete programme to be carried out in Netherlands.

First weapons were shipped back to Canada late in 1995 and all work has now been completed.

All of the barrels for these conversions were supplied by Royal Ordnance Nottingham.

Chile has more recently awarded Royal Ordnance a contract to upgrade a substantial number of M101 s and details of this contract are given later in this entry under the RDM Technology/Royal Ordnance teaming arrangement.

The major modification includes the installation of a new Royal Ordnance 105 mm 33 calibre barrel with progressive rifling and a new muzzle brake. The M101 is also modified in a number of other areas including the incorporation of larger capacity equilibrators, new metering and recoil rod in the recoil system and new seals, replacement of the original elevating arc and reinforcement on the drawbar.

The new 105 mm 33 calibre barrel is ballistically similar to the latest generation of 105 mm artillery ammunition and is suitable for high pressures, with the design being for 4,500 bars.

It has progressive rifling (1.35 to 1.18) over a length of 2,794 mm whereas rifling is constant (1.18) over the last 306 mm (muzzle end). The barrel has 36 lands and grooves and provides a shot travel of 3,100 mm with a chamber volume of 2,507 cm2. This is identical to the standard M101, allowing the upgraded weapon to fire all existing types of 105 mm ammunition.

The modified breech mechanism is still of the horizontal sliding type, but due to the higher forces generated on firing, the original breech ring is replaced by a stronger one which allows it to use all modern ammunition types.

The remaining parts of the original breech mechanism can still be used including the firing mechanism. In order to reduce the increased load on trunnions and recoil when using new ammunition, a three-slotted muzzle brake is fitted to the threaded end of the barrel. This muzzle brake provides for a substantial reduction in the recoil forces while still maintaining blast overpressure within acceptable limits.

A modification to the recoil system was required due to the increase of the recoil momentum, resulting from the use of the 33 calibre barrel and more modern ammunition. The increased recoil momentum is absorbed partly by the muzzle brake and partly by the recoil mechanism. The original control rod with the recoil cylinder is replaced and tailored with respect to the increased recoil momentum of the heavier cannon. To limit recoil the diameter of the recoil cylinder is increased. The recoil distances have not been changed and are still the same as the original. The original seals are replaced in order to retain the larger fluid pressure generated during the recoil.

As the original two-spring equilibrator is unable to compensate for the increased tipping movement, a new equilibrator has been mounted in such a way that it now houses three nested springs instead of the original two. The new equilibrator reduces the weight moment at all angles of elevation.

The cradle system does not need to be changed, although a small modification is required in order to connect the new equilibrator. No modifications are required to the carriage of the M101. A small reinforcement on the drawbar connection of the trails is required as well as reinforcement of the trails.

The original M101 howitzer provides for a maximum elevation of +65°, but in order to increase the overlap in ranges the original elevation arc has been replaced which allows elevation to +70°.

Early in 1994 it was announced that Royal Ordnance of the UK had teamed with RDM Technology to offer a number of upgrade packages for the 105 mm M101/ M101A1 towed howitzer.

Marketing lead for projects will be undertaken by either country according to perceived strength within any given market. Generally, Royal Ordnance will be responsible for the ordnance and RDM Technology for the carriage.

The original RDM upgrade, and the one selected by Canada, uses a 105 mm 33 calibre barrel, but now an additional two upgrades are being offered, one with a 30 calibre ordnance which is compatible with the Royal Ordnance L119 Light Gun and the second with a 37 calibre ordnance which is compatible with the Royal Ordnance L118 Light Gun.

The105 mm 30 calibre allows the standard US family of M1 ammunition to be fired to a maximum range of 14,200 m, or using the M913 rocket-assisted projectile a range of 19,500 m can be achieved. Firing the Eurometaal ERM1 (Extended Range M1) base bleed projectile a range of 18,500 m is achieved.

The 105 mm 37 calibre version allows a standard RO L31 high-explosive projectile (originally developed for the Abbot self-propelled gun) to be fired to a maximum range of 17,200 or 20,500 m with an extended range high-explosive projectile.

The RO L31 high-explosive projectile has 25 per cent greater lethality than the old M1 which was developed many years ago. The 37 calibre version has a larger 5 litre chamber which gives a stretch potential for new ammunition in the future.

The prototype of a Brazilian 105 mm M101 system upgraded to the M101/37 calibre system has now been successfully demonstrated with 100 rounds being fired, including Abbot charge super at high temperature.

Full inspection after the UK trials revealed no strength problems and the muzzle blast overpressure was acceptable.

Late in 1997, Royal Ordnance Nottingham was awarded a contract for an undisclosed amount by FAMAE Ordnance Ltda of Chile for the supply of a large number of upgrade kits forthe old US supplied 105 mm M101 towed artillery systems deployed by the Chilean Army.

This is the first upgrade contract for which Royal Ordnance has been selected as prime contractor since signing the co-operation agreement with RDM Technology of the Netherlands covering the joint marketing of the RDM designed upgrade.

Under the terms of the contract, Royal Ordance will supply the 105 mm 33 calibre barrels to RDM Technology, the main subcontractor, who is supplying the other parts including new muzzle brake and breech ring, larger capacity equilibrators, new metering and recoil rod in the recoil system and new seals, replacement of the original elevating arc and reinforcements for the draw bar and trails. The first six complete kits were sent to FAMAE late in 1998, who will upgrade the actual weapons under the supervision of staff from RDM Technology. FAMAE will also supply some components for this upgrade. Once the weapons have been assembled and tested FAMAE will deliver them to the Chilean Army.

The upgraded M101 fitted with a 33 calibre barrel will fire a standard High Explosive (HE) M1 projectile to a maximum range of 11,900 m, or an Extended Range M1 (ERM1) projectile with R0663A2, zone 2 charge, to a maximum range of 19,200 m.