This should be a good shoot on sight deterrent:
Remove all ranged firearms and increase the availability of pistols. A player with a pistol will be able to easily kill another lone player if he can sneak up on him, however a group of 2-3 is far riskier and therefore a deterrent. And if there was no way to snipe players from 100+m with a rifle most kill on sight would disappear because of the added risk of close combat.
I ll happily lose the mosin in exchange for a better shoot on sigth deterrent and some pistols.
Because the developers fail to see the glory of nametags (which would add consequence to KOS), no available rifles would be a softer deterrent because there is far more added risk opening fire on 3 axe wielding players on 20m range than it is with a rifle 200m away.
Making rifles only rarer wont work. No matter how rare mosins are, there will be at least one guy sitting on a default sniper spot per server.