I thought I'd shed a little light on the situation using some real-world experience. I'm an old OFP guy, and am loving ArmA as it is =) Anyway, my "real-world" experience is several years as a military EOD (Explosive Ordnance Disposal) Tech; if you don't know about us, we are responsible for getting rid of all explosive hazards on the battlefield (UXOs, IEDs, etc.) and thus, receive a LOT of training on different types of munitions and their functions.
A HEAT (High-Explosive Anti-Tank) round comes in all shapes and sizes, and are almost always direct-fire weapons. The HEAT round uses the Monroe effect to focus an explosive shock-wave against armor - the so-called "shaped charge."
More information can be read up on here if anyone is interested - http://www.globalsecurity.org/militar....rge.htm
Basically, in order for a HEAT munition to be effective, it must have some amount of stand-off from the target when the charge detonates, or the wave will not have time to form, and the weapon will fail. This is why HEAT munitions are generally direct-fire only - they have to actually impact their target, not just hit near them. As far as HEAT from artillery, the only exception to the rule AFAIK is a cluster munition, for instance a 155mm projectile that dispenses several hundered tiny bomblets which employ HEAT charges to impact the target from above as they drift downward.
I continue to hear people discuss HEAT as an anti-personnel weapon when OFP/ArmA is concerned, and while this may be effective in the game, the blast radius of a tank's HEAT projectile is going to be relatively small, unless the round was specifically designed to also fragment its casing as an AP effect.
The other option for firing from a tank's main gun in OFP/ArmA is the sabot round, or APFSDS (Armor-Piercing Fin-Stabilized Discarding-Sabot). These weapons function purely on kinetic energy, and contain no explosive charge whatsoever. They are typically constructed of an ultra-dense material in order to penetrate as much armor as possible. The sabot does not actually set off any explosion inside a vehicle it strikes, it's effects are much more gruesome. If successful, a sabot round penetrates the tank entirely, entering one side and exiting the other. Due to the extreme velocity of these projectiles, this causes a rapid pressure change inside the vehicle and creates a vaccum inside the cabin. This, like a rapid decompression of an aircraft at high altitude, tends to suck everything possible (crew included) out the small (4-6 inches diameter) exit hole on the armor, thus killing crewmembers with both fragmentation from a spalling effect on the interior of the armor, the passing shockwave of the projectile, and the vaccum created inside the vehicle. It also tends to detonate fuel and other explosive components as they are violently strewn about the cabin and forced out as well.
I hope this information sheds some light on the subject, and I hope I haven't bored everyone to death with this bit of technical mumbo-jumbo. If anyone has more questions related to munitions, etc, feel free to send me a PM or ask me in the thread.
