Hank, that is a very good question indeed and I apologize for being a little slow keeping up with the posts. Stupid day job!
There is a method to the madness and let me outline it briefly. As a matter of design I wanted to be able to rate everything from a well-thrown rock to a thermonuclear event on a single easily comprehensible scale, preferably using numbers from 0 to 15 or so. Easy you say? If you use a linear function you just get everything rated 0 -2 or so, and then an H bomb sits up at 15. Big gap in the middle. Not very useful at all. Playtesters complain.
What I went to was a semi-log scale related directly to effective armor thicknesses. I'm not sure that I covered all of my intended range, but it did serve to give a rational basis to this part of the game. The full scale is attached as a jpg. A "1" for example is the equivilent of 1 to 9 mm of standard modern steel armor at the perpendicular. A "2" is 10 to 20.4mm, and so on. Note how the number of mm within the range widens out considerably - that is the semi-log part. Actually I used a power of 1.3 rather than 2 to scale it just so, but you get the idea.
Armor with a value of "11" in this game is rated to the equivilent of 352 to 462mm of standard armor. Yes, I am well aware that some parts of a Challenger are thicker and others thinner, and that the slope of the armor counts too, and it is Chobham armor anyway (I convert it as best I can to the equivilent of steel), and some armor is reactive (handled using separate rules), and that tanks have numerous weak spots that constitute critical hits, and that it all really depends on the facing of the tank (not tracked in FPG) versus the incoming shot, and besides these numbers are all still classified anyway. My best information is that if you average all this out you will fall into the 352 to 462mm zone for a Challenger. I could be wrong of course but if you search around on the web on the topic of effective armor thicknesses you will find that there is surprising disagreement on how it is all calcuated and how poorly battlefield results can correlate to manufacturer's claims.
If we have the protection ratings then how do the attack ratings work? Well, they are on the same scale. My assumption is that if you are rated an "11" (352/462) and you are hit by a "12" (463/605) then in the overwhelming number of cases you will be mission killed. I suppose there are freak cases where the shot would hit but not penetrate for some reason and so you cannot say it is absolutely 100% but it would be pretty darn close in the great majority of cases.
What is if you are hit by a round rated "11"? Well, you are both in the same band of mm and therefore it is pretty much of a tossup whether the round can penetrate and kill or not. It could all depend on a host of surrounding circumstances like target aspect. In a micro-tactical game we can model all this, but in FPG there is a flat 50% chance of kill when the attack rating = the protection rating.
If the attack is less than the protection rating then the chance of kill falls away steeply, but it still exists thanks to critical hits, rear shots, defective armor, etc. A -1 differential has a 25% chance of kill (lucky hit), a -2 is 12.5% (lucky critical hit), a -3 is 6.3% (you get the idea), -4 is 3.1% and so on. It is possible for a small calibre gun to mission kill a main battle tank (perhaps it broke the track, or killed the commander who was looking out his hatch, or carried away a crucial radio mast, or jammed the turret ring, etc) but the chances aren't very good and it is not something that you should count on.
FWIW, most WP arty fire against armor falls into this forlorn hope department. The individual odds are poor but if you fire enough arty then you will get some kills. NATO arty has a mix of ICM automatically thrown in so it does much, much better.
So now that you know this you can go back to the platform ratings and see at a glance but the basic chances of kills are between various combinations. The other circumstances of the duel will influence the results of course, but the basic chance of a kill is defined this way.
'Chance of a kill' is just 1/3 of the duel resolution. First you have a 'chance of sighting' that must be successful, then 'chance of hit'. These get a little complicated and will have to wait until my boss in not wandering around.
Cheers, Rob.
