Balance: Reduces raw damage from Tranq rounds (#3900)

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# About the pull request
Reduces /datum/ammo/bullet/pistol/tranq brute damage from 40 to 15
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# Explain why it's good for the game
As of the moment the tranq pistol bullet does more actual damage than
stun damage, and can kill a marine in 3-4 shots, which is as many as
needed for the stun to trigger anyway.

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# Changelog

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:cl:
balance: Reduced tranq pistol bullet damage from 40 to 15
/:cl:

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code/modules/projectiles/ammo_datums.dm

@@ -309,6 +309,7 @@
 	name = "tranquilizer bullet"
 	flags_ammo_behavior = AMMO_BALLISTIC|AMMO_IGNORE_RESIST
 	stamina_damage = 30
+	damage = 15
 
 //2020 rebalance: is supposed to counter runners and lurkers, dealing high damage to the only castes with no armor.
 //Limited by its lack of versatility and lower supply, so marines finally have an answer for flanker castes that isn't just buckshot.