Ah, yep, so you're trying to change the gun of a 'main' character, rather than your average AI, so they are more likely to have their weapons defined in the level scripts, usually some point after they have spawned in the game (so changing their .tik file is useless).
in maps/e2l2.scr, for example, there is an initPlayer function called on map start which changes the AI's gun (first line). In this case the AI has the (target)name of lyndon. The dollar before the name represents a targetname operator. The string that follows it is their targetname. In this case, the name that was given to them in Radiant.
You can change the weapon in the quote to thompson to change his gun at start. However, as mohaa SP is heavily scripted, especially friendly AI, it is often the case that the weapon is given to AI multiple times in a mission. In fact, quite often the character you see in one scene is actually a new AI in another, as the originals can be despawned when not in use. I did a 'find in files' for the e2l2 scripts and couldn't find any more references to 'vickers' so you might be safe changing that one line.
//------------------------------------------------------------------------------
initPlayer:
//------------------------------------------------------------------------------
$lyndon gun "vickers" <-------- change this
exec global/loadout.scr
thread global/items.scr::add_item "explosives" noprint
$allhell_vehicles nottriggerable
$spawnlyndon nottriggerable
//init the warehouse door
$warehousedoor sound_open_start gate_iron_open1
$warehousedoor sound_open_end door_metal_open_stop1
$warehousedoor sound_close_start gate_iron_open1
$warehousedoor sound_close_end door_metal_open_stop1
end
For BT, you will find the main script in maintt/maps folder (pak1.pk3), and there are subsequent 'gag' scripts residing in maps/<mapname> folder that typically control each scene in a given level. As you can see, you will find the scripters/designers rarely put code in the right place. initPlayer is a terrible name for a function that changes more than the player.