1. multiplayer_server_agent_hit_common trigger in mission_templates
In block which applies to server ( (this_or_next|multiplayer_is_server), (neg|game_in_multiplayer_mode),)
-Get victim's agent (store_trigger_param_2,":attacker_agent_no"),
-Check if attacker is a player, check if victim is a player (if you don't want this to work for TKing bots), get their players (agent_get_player_id,":attacker_agent_player",":attacker_agent_no"), and their teams and compare if they're same team to continue
-Check if victim will lose more or equal to HP that he already has (if you want this to work only for killing and not wounding) to continue. You get damage dealt with (store_trigger_param_3,":damage"),
-Get attacking item stored in reg0 and check if it is firearm, or a bullet, I'm actually not sure what kind of item will be attacking item in case of firearm.
-if all checks are true, (kick_player, ":attacker_agent_player"),
2. Not sure what kind of message you want to display, look into how script_multiplayer_broadcast_message works.