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Defence Technology — March 2004
Does Canada have alternatives to the Mobile Gun System?
Tom Rea considers the importance of compact
components and the use of mass-efficient armour in the design of modern Armoured Fighting Vehicles
MGS Sidebar – 155mm Main Armament “... seeking whom he may devour
...”
The development of a new large-calibre weapon is often a highly expensive and lengthy undertaking. This need not
be the case. Generally, the barrel – or ‘tube’ – of the new weapon must be capable of
withstanding much greater pressures than its predecessors. As artillery shell propellant loads become more powerful,
the strength of any new artillery piece 'tube' must likewise increase. One solution to this
developmental spiral is to use a low-pressure weapon in a larger calibre.
Using a larger calibre, in this case 155mm, allows the low-pressure armament to at least maintain the
performance of a smaller high-pressure gun. As already noted, the L7 (M68A1) 105mm gun mounted on the
LAV III-based Mobile
Gun System is considered obsolete – 120mm weapons are now the NATO standard. Our 155mm
low-pressure gun must match the performance of a high-velocity 120 to be viable.
One economical method for developing a ‘new’ low-pressure 155mm calibre gun would be something of an
exercise in recycling. Stocks of surplus 105mm L7 guns are available (hence their ill-considered use on the
MGS) as are 155mm tubes from various field gun and howitzer re-barrelling programs.[1] A new low-pressure gun
could be devised by the simple expedient of marrying the breech assemblies and concentric recoil systems of
L7s to the cast off tubes from those re-barreled 155s. Since this is to be a low-pressure gun, the tubes
could be fluted to reduce weight.
The 155mm gun would be mounted in a flush, centrally-mounted, fully automated turret. Ammunition stored
vertically in the turret basket would be retrieved by an auto-loader which then feeds the round to the breech.
(The main armament would be interchangeable with a high-pressure artillery piece which would employ the same
ammunition and auto-loader.) The defensive armament (comprised of twin remotely-controlled
machineguns) would also rotate with the turret basket.
The result would be a powerful, hard-hitting weapon. The wide bore of the 155mm gun permits its primary
armour-piercing round to be compact in length (compared with equivalent 105mm and 120mm rounds) permitting
vertical stowage of rounds in the turret. Of course, the gun would not be limited to armour-piercing
rounds – any of the full range of artillery shells developed for 155mm calibre could be used. Indeed,
the large bore also makes it practical to consider firing guided munitions.
Gun-fired guided munitions ran into technical problems in tank guns of 105mm to 125mm – most springing
from the difficulties of squeezing components which are tough enough to survive high muzzle-velocities into a
relatively small bore. But, neither technical hurdle is present in a low-pressure 155mm gun. Guided
anti-tank munitions have obvious advantages over conventional rounds but, there is also potential for
anti-aircraft rounds which could provide unparalleled protection. [2]
It would be possible to further expand the tradition role of AFV main armament by adapting other types of
artillery rounds to 155mm calibre. Mortar shells, possibly derived from the existing 160mm types, would have
greater range when fired from the 31 calibre tube. This added range, and a high rate of fire, would make the
low- pressure 155mm ideal for providing close-support for infantry. Mortar shells fired as demolition rounds
would also be very well suited to combat in built-up areas.
To make full use of the effect of 155mm mortar and artillery shells, the AFV’s gun would have to be
capable of high angles of elevation. The plunging fire of mortar and artillery shells could inflict heavy
damage on tanks or any other force if well directed. Taking advantage of network-centric sensors and
communications, our AFVs could exploit their low, terrain-hugging silhouette. Raining down powerful
armour-piercing rounds onto thin top-armour would destroy even the heaviest of tanks. The AFVs could then
engage directly, finishing off anything left intact or only partially destroyed. This dual direct-fire and
indirect-fire capability – which tanks lack –
could be exploited under any terrain conditions or tactical situation.
This 155mm low-pressure weapon would out-gun almost anything it encountered on the battlefield. However,
this large bore gun also has future growth potential. Increasing muzzle velocities would be an obvious
step. The low-pressure 155mm, as devised, would be limited to maximum muzzle energies of 2400 ft-tons but there
is room for improvements in muzzle velocity. That recycled L7 recoil system has some reserve. If
that were combined with an energy attenuating system such as RAVEN [3], the muzzle energies can be increased to
as high as 3000-4000 ft-tons. But, higher pressure comes at a cost in heavier, stronger materials. Replacing
the tubes with titanium or composite barrels would keep weight growth down to zero.
[1] One example is the 31 calibre tubes originally fitted to the M109 self-propelled 155mm howitzers. These
surplus L/31 tubes are quite adequate for our purposes.
[2] In an emergency, the guided anti-aircraft and anti-tank rounds could be fired recoillessly,
directly from the magazine greatly reducing the firing sequence.
[3] The RAVEN method vents breech gases before the projectile leaves the tube but also before the
pressure-front traveling behind the projectile can be reflected back to the breech end, thus reducing the
recoil forces imposed on the weapon.
< Part 6 — Visualizing How Design Considerations
Might Shape an AFV
> Part 7 — “... eyes they have and see
not ...” Sensor Systems and Survival
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