[Whitetiger]

pawn Код:

new LastShot[MAX_PLAYERS] = -1, Shooter[MAX_PLAYERS] = -1;
public OnPlayerTakeDamage(playerid, issuerid, Float: amount, weaponid) {
    LastShot[playerid] = weaponid;
    Shooter[playerid] = issuerid;
}

public OnPlayerDeath(playerid, killerid, reason) {
    if(LastShot[playerid] != -1 && Shooter[playerid] == killerid) reason = LastShot[playerid];
}

public OnPlayerSpawn(playerid) {
    LastShot[playerid] = -1;
    Shooter[playerid] = -1
}

or

pawn Код:

public OnPlayerDeath(playerid, killerid, reason) {
    reason = GetPlayerWeapon(killerid);
}

I would use the first example cause it fixes a bug if the player changes weapon after firing rpg,nade etc

*UNTESTED CODE*