m13 buff (shiptest-ss13#3839)

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## About The Pull Request
makes the burst delay on the m13 small so it fires in cute little bursts
better

## Changelog

:cl:
balance: m13 larker burst fire speed increased

/:cl:

code/modules/projectiles/guns/manufacturer/serene_sporting/ballistics.dm

@@ -135,6 +135,7 @@ EMPTY_GUN_HELPER(automatic/m12_sporter)
 	name = "Model 13 \"Larker\""
 	desc = "A common after-market modification of the Model 12 \"Sporter\" rifle, keyed to fire a three round burst."
 	burst_size = 3
+	burst_delay = 0.6
 
 	icon_state = "larker"
 	item_state = "larker"