It's not that I don't believe, it's just that I don't recall that the ability of a HEAT warhead to penetrate armor has anything to do with heat or by being "plasma-fied." Sure, once penetration has been achieved, the spall from the penetration would have nasty effects, but that's not how HEAT *defeats* armor, from my understanding. It's just something I picked up on the Tanknet forums, which is probably the number one source of information on this stuff.
Your brother also probably shot an APFSDS round at other tanks, not HEAT. I assume the targets were Iraqi T-55/72s? Not that it invalidates anything of what you or your brother said, but tanks typically don't shoot 120mm HEAT rounds at each other when they have a capable APFSDS round, like the M829 series in the U.S. or the DM53/63 if you're driving Leopards. Both types of rounds rely on a penetrator to defeat armor - it's just that HEAT rounds use a "plasmified jet" formed by an explosive shaped charge as its penetrator.
I may be remembering wrong, and if I am, I apologize to Tanknet, but I don't think so. For our purposes, a HEAT round can be treated as a solid projectile.
Here's a thread on the subject: http://208.84.116.223/forums/index.php?showtopic=18117
Refer to Post #3.
I'm not huge on chipping in my 2 cents on accuracy/spread figures. Mostly because I feel I need to actually shoot the gun in New Vegas before I can comment. Not because your figures are unrealistic, or whatever, but because it's a game and balancing is delicate - I prefer to "feel" how the guns shoot at range.