Each member of an APC crew and each embarked infantryman needs about one cubic metre of protected volume. With additional space for movement that becomes more than 10 cubic metres for a minimum size crew of driver and commander and a squad of eight, and makes the squad-carrying APC a quite bulky vehicle. Some APCs have upper and lower sides that are inward sloping but a sculptured and confining bodyshape such as on the 1950s Alvis Saracen has become uncommon because it does little to conserve the area and weight of armour.

To protect its crew and mounted infantry with their equipment - and separately the engine compartment and fuel tank(s) - the armoured surface area of an APC is typically more than 40 sqm. Hence within the light category it is feasible to shape and protect a modern light APC to all-round S-4569 Level 3 with homogeneous steel armour and to obtain a somewhat higher protection for its underbelly and over its frontal aspect. Getting to all-round Level 5 with homogeneous armour protection is impossible for a light APC but almost possible for a medium APC.

Higher general levels of protection can be passively achieved by use of composite layered armour and spaced armour. These armours can have their outer surfaces augmented by ablative arrangements and explosive reactive armour (ERA). A major problem with ERA is that it cannot be de-activated before infantry dismount. Also stand-off protection against shaped charge projectiles can be extended by attaching see-through steel or aluminium screens with close-set bars or slats. Using modern passive armour techniques - and without resort to ERA - the general protection provided to a medium vehicle of APC size can be raised above S-4569 Level 5, and the forward part of the hull can be further protected to withstand threats such as 30mm cannon fire.

Research has been conducted into reactive electric armour but without apparent employment to date. Research into active/passive defensive aids suites has been more successful. Numerous suites are being developed and some have been installed to provide early-warning and intercept or disruption of various types of anti-armour threat. However, many such systems are problematic because they are signature dependent. Also a wide spectrum suite can be costly and provide only limited multi-shot protection. Use of ERA and defensive aids radiation and interceptors can also be hazardous to dismounted infantry. But - unlike ERA - defensive aids suites or selected sub-systems can be deactivated whenever appropriate.

An APC needs a defensive armament for use against ambush and for local security when in overwatch and laager. That armament might also be employed offensively while infantry dismount. Stabilisation is useful but increases weight, complexity and cost. And it is appropriate to emphasize that notwithstanding its armament an APC is mainly intended for use as CS vehicle rather than for routine employment as a lightly-armed AFV. With protection and mobility as its main attributes it is apparent that the appropriate defensive/offensive weapon or weapons for an APC should be unobtrusive in terms of in-board volume and length, and that the weight of each weapon and ammunition should also be constrained.

A one-man 360 degree rotatable cupola or turret can provide top-mounted all-round viewing by the vehicle commander. If located near the front centreline of the cabin the increase in cabin length might be reduced below 0.7m but could still equate to suppression of probably two infantry seats. A location offset to one side anywhere down the cabin can similarly mean that head clearance will not be obstructed by cable runs. And an offset 1-man turret can be accommodated in little more than the space needed for one infantry seat. The ‘second’ seat in a 2-man turret was sometimes promoted on the basis that it could be interchangeably used by an infantry squad commander to see outside the vehicle. However, episcopes and electro-optics such as CCTV now enable such viewing by infantry seated anywhere in a vehicle.

Development of effective systems for operation of overhead weapons and skeletal RCWS mounts has been accompanied by a trend to remotely operated turrets. With ready magazine(s) included in the turret the need for bulky trunking is reduced. However the turret itself tends to become bulky and there is still a need for manual or mechanical replenishment of magazines from lower in the hull. Provision for head-up – but not simultaneous all-round – direct viewing by a vehicle commander and/or gunner and also manual operation of any ancillary external weapon such as a MMG can then be arranged by means of roof-mounted hatches or cupolas forward and aft of the turret. Overall an unmanned turret is more suitably mounted on a medium than on a light APC.

Alternatively a weapon can be in an external mount above a mini-turret or cupola or independently mounted as a RCWS. Both those types of overhead mount avoid in-board concerns such as bore evacuation and venting, but magazine arrangements and clearance and reloading of overhead external mounts are more complex than in a turret. And for a RCWS, there is the further difficulty that its pedestal must include the traversing and elevating mechanisms that can for an overhead mount be included as part of a mini turret or cupola.

Similar to the 1-man turret on a light APC, the cupola or mini-turret or unmanned turret on a medium or heavy armoured vehicle can be offset to one or other side of the roof centreline. And to reduce the intrusion of trunking or cable runs any 2-man turret or large unmanned turret might be installed near the forward bulkhead of a troop cabin

For head-up operation on a light, medium or heavy vehicle, the vehicle commander or gunner may also be provided with a flexible MMG or AGL mounted on a swing-arm or a ring and skate around a hatch or an otherwise unarmed cupola.

Light APC – mechanised infantry

Initially there is the question of wheeled or tracked. The important difference is that of operational or tactical mobility. The army of any large industrialized state will tend to have both wheeled and tracked light APCs, with the wheeled APCs concentrated in cavalry/reconnaissance and early entry units and the tracked APCs used for main force infantry and CS units. Examples of well configured light APCs are the Rheinmetal Tpz-1 Fuchs 6x6 and GDLS Stryker 8x8, and the tracked Steyr 4K7 and - despite slab sides - the UDLS M-113A3 and M-113A4/MTVL.

The largest readily installed and operated defensive/support weapon for a light APC is the 12.7x99mm HMG or - for approximately the same weight and space - a 40mm AGL combined with a 7.62mm rifle-calibre MMG. As a general purpose DF weapon the choice is either a 7.62mm MMG or a 12.7mm HMG. However a 40mm AGL is clearly superior to either for bombardment tasks and also useful for DF and anti-armour fire. Hence instead of a single HMG, a combined armament of AGL and MMG would provide a vehicle commander with useful choice. Due to their different characteristics it would not be practicable to use both weapons simultaneously, and concern to keep one or other weapon in use at any particular time would reduce the risk of overtasking. For flexibility also an AGL/MMG combination is preferable to the short-term prophylaptic benefits of twin MMGs and the complexities of an alternate rotary MG.

So an appropriate armament for a light APC could have a 40mm AGL and MMG in a 1-man turret. If a capability for hull-down fire were rated as essential, then somewhat less satisfactorily the MG could be on an overhead mount carried above a 1-man turret mounting the higher trajectory AGL whose feed and re-loading/replenishment is also more problematic. Regardless of the increase in weight and despite its cost, stabilisation of the armament for elevation/range should be rated as essential with full stabilisation rated as desirable.

A more powerful armament would have a 12.7mm HMG combined with a 40mm AGL. However, an AGL/MMG combination provides more flexibility for counter-ambush and harassing fire than the HMG/AGL and also the HMG/MMG alternative. Additionally a 40mm AGL and a MMG can be more readily dismounted and deployed on tripods than a 12.7mm HMG that weighs about 40kgs plus the weight of its tripod. Also the 12.7mm HMG with its ammunition may not already be in service with a
supported infantry unit. Finally, arming an APC with all three weapons would overtask the commander in a 2-man crew and hence necessitate a third crew member and an additional cupola or turret. The result would over-crew a light APC and increase its weight and bulk.