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The Bofors QF 40mm cannon is an anti-aircraft gun repurposed (much as the german 88 gun was) into a ground-attack cannon. Its general effect is reminiscent of the german Wirbelwind-Flakpanzer: a high-caliber, rapid-firing gun, with a small explosive power.
The gun can turn a full 360 degrees at a moderate speed, easily catching up to flanking infantry. Fast-moving vehicles are difficult to acquire, but will take extensive damage. The gun will wipe out kubelwagens in the blink of an eye, and will cause extreme damage to Pumas and half-tracks, probably destroying them before they can be moved back to cover. Stugs will be able to hold up for a while in a fight, but Panzers will take heavy damage, except if they have the flank armour pannels, which are extremely effective at reducing the Bofor's damage. Any heavier armoured vehicles will take almost no damage from this gun.
When in the game should I build Bofors? Edit
In the late early game, if the enemy has not fielded mortars, or if you are floating unseemingly amounts of manpower and they could not possible be spent elsewhere (they really, really could). The Bofors will make the enemy pressure vanish from the sector, but players that build it need to be aware that they have essentially spent 280 manpower to buy a few minutes to prepare. If it's construction is not immediately followed up by auxiliary defence measures, the player has just sent 280 manpower down the drain.
The late game is not the right moment to build Bofors: the enemy has already probably amassed artillery that will rip the emplacement to pieces, and their tanks are no longer affected by what damage the Bofors can still dish out.
Similarly, building a Bofors too early might stimulate the enemy to build mortars, which will wipe the Bofors out of existence Repeat: mortars are extremely effective against Bofors, since they are large, static targets with no roof to protect either gun or garrison.
Where should I build my Bofors? Edit
Bofors should ideally exploit their 360 degree turning radius, so make them cover intersections. Furthermore, do not put them within grenade-lobbing distance of a corner: infantry might be able to dart out, throw something, then retreat, making you the player very, very, very frustrated. And Bofor-less.
The ideal fortification Edit
The ideal fortification is attacking infantry and tanks, and the element of initiative.
However, if you really, really want to go on the defensive, a lone Bofors will not be enough. It should be supported, first by 3-inch mortars to take out nearby artillery threats, and a 17-pounder to neutralize larger enemy tanks. Another Bofors could be added to minimize the effect of flanking enemy elements. A single lucky bomb, or just a well-co-ordinated enemy attack, however, will easily overcome this fortification.