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Coaxial machineguns are machineguns that point the same direction as a tank's (or IFV's) main gun. This makes aiming simple and the vehicle needs no additional means for training and elevation of the machinegun.
Coaxial machineguns use the mass of the turret for cooling and can be reloaded from under armour. Their biggest downside is that most coaxial machineguns allow burnt propellant gases to pollute the air within the turret, but that's an even bigger issue with the main gun, so ventilation is a must in crewed turrets. Coaxial machineguns have been common since the 1930's.
A special (and rare) kind of coaxial machineguns are retrofitted heavy calibre machineguns that are mounted outside of the turret.
You've very likely read about the threat of remotely-controlled flying drones to tanks. They're radio command-controlled and susceptible to jamming, but few tanks have such jammers. Netting is used to counter such drones, but purpose-built fuses would easily counter any such net.*
The answer to such drones is mostly jamming, though you may also shoot them down or burn them with lasers. Jamming can be done with quite simple means; you merely need power supply, a radio transmitter for the correct radio frequency band and a (directional) antenna that fits said band as well.
The whole package can be compact enough to be a one-man 'weapon'.
It's obvious that Western MBTs and IFVs are not prepared to deal with such a threat, and they are VERY vulnerable to it. We could equip them with jammers with omnidirectional antennas, but the permanent emissions by such antennas would be very easily triangulated and inform the enemy about tank locations and movements. We could switch such jammers on only when needed, but this requires the knowledge when they are needed; the detection of the drone.
You may use a weaker (or at same output power more effective) jammer with a directional antenna when you know where the drone threat is. So there's a case for directional drone jammers to be added to armoured fighting vehicles.
We already have quite a garden of antennae, cupolas, sensors and guns on top of tank turrets, though. A coaxial installation of a drone jammer antenna may thus be the way to go IF one decides against an omnidirectional antenna jammer. It may also make sense to have omnidirectional self-protection jammers and one tank or infantry fighting vehicle per platoon equipped with a longer-ranged directional jammer. The longer ranged one would be against observation drones, while the self-protection jammers would only affect the much more close attack drones.
I didn't write much about such jammers in the past because I consider remotely-piloted vehicles as a transitory thing. The really big deal will be drones with a degree of autonomy that allows them to do their job without an intact two-way datalink with a human operator.
My preference remains the use of mass-produced standardised remotely-controlled weapon stations (RCWS) on almost all battlefield vehicles (80+ % of the vehicles of a mechanised infantry brigade, for example). I hope we can make do without onboard search radars.
S O
defence_and_freedom@gmx.de
*: Drones with simple impact fuse can be countered, but a fuse can be built with an acceleration-measurement chip that sense the sudden deceleration when the drone gets caught by the net or cage and initiates the shaped charge explosion. The result would be a shaped charge attack with near-optimum standoff distance; even worse than a textbook impact fused shaped charge attack.
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