Defines:
#define DAMAGE_COLT 20
#define DAMAGE_SILENCER 20
#define DAMAGE_DEAGLE 40
#define DAMAGE_SHOTGUN 70
#define DAMAGE_SAWNOFF 50
#define DAMAGE_UZI 18
#define DAMAGE_MP5 18
#define DAMAGE_AK47 20
#define DAMAGE_RIFLE 45
#define DAMAGE_TEC9 18
#define DAMAGE_M4 20
#define DAMAGE_SNIPER 50
#define DAMAGE_KNIFE 20

Code:

public OnPlayerTakeDamage(playerid, issuerid, Float:amount, weaponid)
{
    new damage = 0, Float:x, Float:y, Float:z, Float:distance = 0.9, Float:health, Float:armour;
    switch(weaponid)
    {
		case 4: damage = DAMAGE_KNIFE;
        case 22: damage = DAMAGE_COLT;
        case 23: damage = DAMAGE_SILENCER;
        case 24: damage = DAMAGE_DEAGLE;
        case 25: damage = DAMAGE_SHOTGUN;
        case 26: damage = DAMAGE_SAWNOFF;
        case 28: damage = DAMAGE_UZI;
        case 29: damage = DAMAGE_MP5;
        case 30: damage = DAMAGE_AK47;
        case 31: damage = DAMAGE_M4;
        case 32: damage = DAMAGE_TEC9;
        case 33: damage = DAMAGE_RIFLE;
        case 34: damage = DAMAGE_SNIPER;
    }
    if(damage != 0 && !IsPlayerInRangeOfPoint(issuerid,distance,x,y,z))
    {
        if(armour > 0)
        {
            amount = armour - damage;
            if(amount < 0) return SetPlayerHealth(playerid,health-damage);
            return SetPlayerArmour(playerid,armour-damage);
        }
        else return SetPlayerHealth(playerid,health-damage);
    }
	if (PlayerData[playerid][pFirstAid])
	{
	    SendClientMessage(playerid, COLOR_LIGHTRED, "[WARNING]:{FFFFFF} Your first aid kit is no longer in effect as you took damage.");

        PlayerData[playerid][pFirstAid] = 0;
		KillTimer(PlayerData[playerid][pAidTimer]);
	}
	return 1;
}