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I have just had a quick pootle with a simple square-law drag program I wrote in Python a while ago, which calculates the angle of fall of a projectile given its initial velocity and drag. Taking one of the most curved-trajectory tank guns I can think of, the 7.5cm KwK L/24, and guessing a drag coefficient based on some curve-fitting to published penetration tables I did earlier for fun, I reckon it would be falling at an angle of about 5 degrees at 2000 metres, and twice that at 3000 metres. Good luck hitting anything at that range; the program gives the time of flight as about ten seconds.
On the strength of this, I do not think the angles are likely to involve any useful element of top attack for any range at which a target is going to be seen, as long as the shot is fired in the lower register. In the upper register, obviously things are going to have more of a coming-down than going-along component, but I am not aware of any tank main armament that can fire in the upper register.
Howitzers can, and mortars usually must, fire in the upper register, and the big advantage of this at the target end is that the lethal area — the intersection with the ground plane of the expanding doughnut of steel fragments from the shell burst — can be a good deal larger for the same shell. If wargames rules writers were such obsessive detail freaks about field artillery as they were for AFVs, there would be rules showing how the lethal area varies with range, but as far as I am aware no wargames rules, and indeed no artillery norms in Russian, British and Canadian artillery doctrine that I have seen, do so. Not much point making the game more detailed than real life, is there?
The dear old 1973 WRG 1925-50 rules I recall used the same areas of effect for both direct and indirect area fire, and this seems quite sensible to me; the problem is that too few rules these days allow for direct area fire. This changes the question to “at what range does point fire become area fire?”, and that is going to depend on the precision with which the targets can be located. At (say) 2000 metres, with binoculars, you may well be able to spot a building or substantial field fortification well enough to engage it with point fire, but for properly-concealed infanteers, forget it. The WRG rules let the firer decide.
The other aspect to consider is how the shots are sensed and corrected. Most WW2 HE weapons, I believe, would employ bracketing drills in order to correct fire onto a target, somewhat after the fashion of an FOO correcting artillery fire. However with the 75mm M3 in the Sherman (and so analogously the 75mm Mk 5 in Cromwell and Churchill) it was found possible to employ direct laying — as if shooting a tank — up to 2,000 yards. That seems to have remained pretty much the British Army’s doctrinal distance for precise tank HE fire in support of infantry ever since; at least, in my day, Chieftain was supposed to do this with 120mm HESH up to 2000 metres.
Mr. Picky, while forgiving, would like to remind people that a “barrage” is a linear pattern of fire (as one might expect from the original French). Most artillery fire on the wargames table is fired in simple concentrations. Nigel Evans’ wonderful artillery pages doubtless have more detail.
All the best,