Looking for a clear explanation...

[Imp]

Good grief I think hes got it :)

Thats why in MBT most times things just go bang big diffrence between the penetration & armour levels.

Infantry AT weapons with the exception of ATRs & Guns are all HEAT so range does not matter. ATRs are button up items or use to get there attention so another unit gets a flank shot. Need multiple shots short range & a bit of luck to do anything even then a destroyed result is super rare.
Always try to be stationary when fire inf AT to maximise hit chance, the only exception is if you can get adjacent to it.
If the player is kind enough to let you get 2 squads buy it unmolested if it does not die that turn (unlikely) probably will the next if they have AT & you position them right.
If your best AT weapon is a hand grenade assault it with your last fire giving the guy with the AT weapon a break when he moves in.