adds a snowflakey-ish(??? maybe???) way to modify gun damage so we don't need more shot types for improvised shotguns (#12274)

* Update gun.dm

* Update _firing.dm

* Update code/modules/projectiles/ammunition/_firing.dm

Co-authored-by: Ghom <42542238+Ghommie@users.noreply.github.com>

code/modules/projectiles/ammunition/_firing.dm

@@ -31,6 +31,10 @@
 	else
 		BB.def_zone = user.zone_selected
 	BB.suppressed = quiet
+	
+	if(isgun(fired_from))
+		var/obj/item/gun/G = fired_from
+		BB.damage *= G.projectile_damage_multiplier
 
 	if(reagents && BB.reagents)
 		reagents.trans_to(BB, reagents.total_volume) //For chemical darts/bullets

code/modules/projectiles/gun.dm

@@ -76,6 +76,9 @@
 	var/datum/action/item_action/toggle_scope_zoom/azoom
 
 	var/dualwield_spread_mult = 1		//dualwield spread multiplier
+	
+	/// Just 'slightly' snowflakey way to modify projectile damage for projectiles fired from this gun.
+	var/projectile_damage_multiplier = 1
 
 /obj/item/gun/Initialize()
 	. = ..()