Adds a rubbershot box to cargo (#3326)

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## About The Pull Request

This should add a box of rubbershot ammo to the outpost cargo market.

## Why It's Good For The Game

This will allow players the ability to purchase rubbershot for training
activities without having to require RnD beforehand.

## Changelog

:cl:
add: Adds a rubbershot box to the outpost market
/:cl:

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code/modules/cargo/packs/ammo.dm

@@ -76,6 +76,12 @@
 	cost = 210
 	contains = list(/obj/item/storage/box/techshot)
 
+/datum/supply_pack/ammo/rubbershot
+	name = "Rubbershot Crate"
+	desc = "Contains a box of twenty-five rubbershot shells for use in crowd control or training."
+	cost = 500
+	contains = list(/obj/item/ammo_box/a12g/rubbershot)
+
 /*
 		.38 ammo
 */