To hide HUD elements like ammo count you need to hook "HideWeapon" message and change it's contents accordingly... what it can hide and the values here:
http://wiki.amxmodx.org/Half-Life_1_...nts#HideWeapon
I'm not sure about this, but you can also make the ammo bar disappear by changing CurWeapon or AmmoX or some other message (I can't remember which) ... search for "sniper crosshair" plugin, that basically changes the ammo to something else so the crosshair appears, if you change ammo to knife or some other ammotype/weapon that hides ammo, ammo count wil disappear.
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