For nuts and bolts stuff - you should refer to the Mobhack help, which goes into more details.
For standard military acronyms, like HEAT, HE, VIRSS etc - these are things that wargamers tend to know, being military history/technical buffs generally.
Google is a great help, if you dont know what an acronym is and also
FAS , e.g
Big Bullets
For game details - see the GG section "Unit Information Screen" and the related "Encyclopaedia Screen" as the details of the Info screen are the same as for the encyclopaedia.
(At the end of the weapon table in the encyclopaedia entry for the GG, and also within the table the ammo types are defined, in any case).
Weapons with an AP penetration value can defeat armour, and you need an HE kill value to have effect on soft targets. Anti tank rounds are generally AP, HEAT, and APDS/APCR but HE can have an effect on armoured targets.
To turn a weapon
off see the "Unit Information Screen" section of the GG, table entry "Weapon Info", final paragraph.
To fire an individual weapon only - see the GG "Main Map Window" section which describes the main game action window, then refer to the table of hot keys, entry for "W" key. This brings up a sub-menu which allows you to fire only a specified weapon at the current target. pressing the 1 key fires
only weapon slot 1 etc. For selecting a specific target - see the "T" hot key or the target buton.
Command and control is judged at turn start - not during the turn. If your guy was > 250 hexes away at turn start, and failed a radio check (assuming both had them) then he will be out of CC for that move.
The "H" key takes you to the headquarters menu - it is the "J" key now to go to the units screen, that does not sem to have made it into the documentation. (No idea why I chose "J" - possibly because it has a raised bar making it easy to find, like the "F" key - been a while
!
Cheers
Andy