Adds the XM51 Breaching Scattergun (#5086)

# About the pull request

[Design doc (a bit out of
date).](https://hackmd.io/uVhllc-KQSGcvjhEUw66bw?edit)

The XM51 breaching scattergun is a 16-gauge, pump-action mag-fed shotgun
that utilizes specialized breaching shells. It allows marines to break
through doors and walls alike with ease at a safe distance. Each shell
fires out four groups of pellets, the main one dealing 55 damage and the
other three dealing 10 each. Though it has decent damage, this weapon
performs weakly in direct combat compared to other options and is
entirely designed for a support role. Its maximum effective range is
about 4 tiles.

It takes:
- 1 shot for doors
- 2 shots for regular resin walls
- 3 shots for thick resin walls
- 2 shots for regular walls,
- 4 shots for reinforced walls.
- 4 shots for pylons,

Each XM51 kit comes with a dedicated holster for the weapon, along with
two spare magazines, two boxes of shells and a stock. Being able to wear
the weapon around your waist gives the user a great deal of flexibility
when it comes to their loadout. The stock makes it unable to be
holstered, but it does give it a very devastating two shot burst-mode
that quickly clears out walls.

Ammo for the weapon is a bit scarce but still present in a decent
quantity. Requisitions starts with a single 120 round shell box which
they can purchase more of, along with a couple of spare magazines. More
magazines can be purchased, and ASRS has a chance of sending up spare
ammo as well.

This weapon is based on the [actual M51 breaching
scattergun.](https://avp.fandom.com/wiki/M51_Breaching_Scattergun)

# Explain why it's good for the game

Marines are very cowardly when it comes to pushing. Especially so when
they're facing countless thick resin walls and doors. This contributes
to a stale meta where the best option is to just sit behind cades and
wait for the other team to make a mistake, because why would you risk
dying to take down one resin door? That's not fun for anyone. Marines
have a few ways of making some breathing room for themselves from a safe
distance, with one of the best ones being the U7 underbarrel shotgun.
It's a very good and fun attachment, even putting aside the combat
capabilities it has.

However, it's entirely exclusive to the M41A rifles. Any marine that
wants to experiment with their weapon choice a bit and forgo the rifle
lose one of the best tools they have access to. The XM51 gives marines
another, more effective way to breach resin bunkers. In exchange for the
ease of use, abundance of ammo and good combat capabilities of the U7,
you gain a safer, faster, flexible and more potent tool.

# Testing Photographs and Procedure
<details>
<summary>Screenshots & Videos</summary>


https://github.com/cmss13-devs/cmss13/assets/17518895/6f4441d2-740e-4110-a9a6-32c39c424576

combat capabilities vs the u7


https://github.com/cmss13-devs/cmss13/assets/17518895/c0cb1359-cfd0-4b86-8c77-3885c86c5c8c



https://github.com/cmss13-devs/cmss13/assets/17518895/834d9bbb-3aee-4203-93a7-af40e28b7805



![xm51camo](https://github.com/cmss13-devs/cmss13/assets/17518895/1ada72ac-2b4a-441b-ac26-c0152a0eb62b)

![xm51ammo](https://github.com/cmss13-devs/cmss13/assets/17518895/471c2654-c242-4440-8b67-8e19bad8eb45)

![xm51statnostock](https://github.com/cmss13-devs/cmss13/assets/17518895/a672ec8a-3401-4830-a1db-821198dabf91)


</details>


# Changelog
:cl:
add: Added the XM51 breaching scattergun, available in small quantities
as a restricted weapon within Requisitions.

/:cl:

code/datums/ASRS.dm

@@ -66,6 +66,14 @@
 	reference_package = /datum/supply_packs/ammo_shell_box_flechette
 	cost = ASRS_VERY_LOW_WEIGHT
 
+/datum/supply_packs_asrs/ammo_shell_box_breaching
+	reference_package = /datum/supply_packs/ammo_shell_box_breaching
+	cost = ASRS_VERY_LOW_WEIGHT
+
+/datum/supply_packs_asrs/ammo_xm51
+	reference_package = /datum/supply_packs/ammo_xm51
+	cost = ASRS_VERY_LOW_WEIGHT
+
 /datum/supply_packs_asrs/ammo_smartgun
 	reference_package = /datum/supply_packs/ammo_smartgun
 

code/datums/ammo/bullet/shotgun.dm

@@ -311,6 +311,32 @@
 	penetration = ARMOR_PENETRATION_TIER_10
 	scatter = SCATTER_AMOUNT_TIER_4
 
+/*
+					16 GAUGE SHOTGUN AMMO
+*/
+
+/datum/ammo/bullet/shotgun/light/breaching
+	name = "light breaching shell"
+	icon_state = "flechette"
+	handful_state = "breaching_shell"
+	multiple_handful_name = TRUE
+	bonus_projectiles_type = /datum/ammo/bullet/shotgun/light/breaching/spread
+
+	accuracy_var_low = PROJECTILE_VARIANCE_TIER_6
+	accuracy_var_high = PROJECTILE_VARIANCE_TIER_6
+	damage = 55
+	max_range = 5
+	bonus_projectiles_amount = EXTRA_PROJECTILES_TIER_3
+	penetration = ARMOR_PENETRATION_TIER_1
+
+/datum/ammo/bullet/shotgun/light/breaching/spread
+	name = "additional light breaching fragments"
+	bonus_projectiles_amount = 0
+	accuracy_var_low = PROJECTILE_VARIANCE_TIER_6
+	accuracy_var_high = PROJECTILE_VARIANCE_TIER_6
+	scatter = SCATTER_AMOUNT_TIER_3
+	damage = 10
+
 //Enormous shell for Van Bandolier's superheavy double-barreled hunting gun.
 /datum/ammo/bullet/shotgun/twobore
 	name = "two bore bullet"

code/datums/elements/bullet_trait/damage_boost.dm

@@ -1,5 +1,7 @@
 GLOBAL_LIST_INIT(damage_boost_turfs, typecacheof(/turf))
 
+GLOBAL_LIST_INIT(damage_boost_turfs_xeno, typecacheof(/turf/closed/wall/resin))
+
 GLOBAL_LIST_INIT(damage_boost_breaching, typecacheof(list(
 	/obj/structure/machinery/door,
 	/obj/structure/mineral_door,

code/datums/supply_packs/ammo.dm

@@ -240,6 +240,16 @@
 	containername = "\improper shotgun flechette crate"
 	group = "Ammo"
 
+/datum/supply_packs/ammo_shell_box_breaching
+	name = "Shell box (16g) (120x breaching shells)"
+	contains = list(
+		/obj/item/ammo_box/magazine/shotgun/light/breaching,
+	)
+	cost = 40
+	containertype = /obj/structure/closet/crate/ammo
+	containername = "\improper shotgun breaching crate"
+	group = "Ammo"
+
 //------------------------For 88M4 ----------------
 
 /datum/supply_packs/ammo_mod88_mag_box_ap
@@ -296,6 +306,18 @@
 	containername = "\improper M41AE2 HPR holo-target magazines crate"
 	group = "Ammo"
 
+/datum/supply_packs/ammo_xm51
+	contains = list(
+		/obj/item/ammo_magazine/rifle/xm51,
+		/obj/item/ammo_magazine/rifle/xm51,
+		/obj/item/ammo_magazine/shotgun/light/breaching,
+	)
+	name = "XM51 Ammo (2x mags) (1x small breaching shell box)"
+	cost = 20
+	containertype = /obj/structure/closet/crate/ammo
+	containername = "\improper XM51 ammo crate"
+	group = "Ammo"
+
 //------------------------Smartgunner stuff----------------
 
 /datum/supply_packs/ammo_smartgun_battery_pack

code/datums/supply_packs/weapons.dm

@@ -60,7 +60,18 @@
 	)
 	cost = 30
 	containertype = /obj/structure/closet/crate/weapon
-	containername = "MOU-53 Breack Action Shotgun Crate"
+	containername = "MOU-53 Break Action Shotgun Crate"
+	group = "Weapons"
+
+/datum/supply_packs/xm51
+	name = "XM51 Breaching Scattergun Crate (x2)"
+	contains = list(
+		/obj/item/storage/box/guncase/xm51,
+		/obj/item/storage/box/guncase/xm51,
+	)
+	cost = 30
+	containertype = /obj/structure/closet/crate/weapon
+	containername = "XM51 Breaching Scattergun Crate"
 	group = "Weapons"
 
 /datum/supply_packs/smartpistol

code/game/machinery/vending/vendor_types/requisitions.dm

@@ -41,6 +41,7 @@
 		list("M2C Heavy Machine Gun", round(scale * 2), /obj/item/storage/box/guncase/m2c, VENDOR_ITEM_REGULAR),
 		list("M240 Incinerator Unit", round(scale * 2), /obj/item/storage/box/guncase/flamer, VENDOR_ITEM_REGULAR),
 		list("M79 Grenade Launcher", round(scale * 3), /obj/item/storage/box/guncase/m79, VENDOR_ITEM_REGULAR),
+		list("XM51 Breaching Scattergun", round(scale * 3), /obj/item/storage/box/guncase/xm51, VENDOR_ITEM_REGULAR),
 
 		list("EXPLOSIVES", -1, null, null),
 		list("M15 Fragmentation Grenade", round(scale * 2), /obj/item/explosive/grenade/high_explosive/m15, VENDOR_ITEM_REGULAR),
@@ -244,11 +245,13 @@
 		list("M41A MK1 AP Magazine (10x24mm)", round(scale * 2), /obj/item/ammo_magazine/rifle/m41aMK1/ap, VENDOR_ITEM_REGULAR),
 		list("M56D Drum Magazine", round(scale * 2), /obj/item/ammo_magazine/m56d, VENDOR_ITEM_REGULAR),
 		list("M2C Box Magazine", round(scale * 2), /obj/item/ammo_magazine/m2c, VENDOR_ITEM_REGULAR),
+		list("XM51 Magazine (16g)", round(scale * 3), /obj/item/ammo_magazine/rifle/xm51, VENDOR_ITEM_REGULAR),
 
 		list("SHOTGUN SHELL BOXES", -1, null, null),
 		list("Shotgun Shell Box (Buckshot x 100)", round(scale * 2), /obj/item/ammo_box/magazine/shotgun/buckshot, VENDOR_ITEM_REGULAR),
 		list("Shotgun Shell Box (Flechette x 100)", round(scale * 2), /obj/item/ammo_box/magazine/shotgun/flechette, VENDOR_ITEM_REGULAR),
 		list("Shotgun Shell Box (Slugs x 100)", round(scale * 2), /obj/item/ammo_box/magazine/shotgun, VENDOR_ITEM_REGULAR),
+		list("Shotgun Shell Box (16g) (Breaching x 120)", 1, /obj/item/ammo_box/magazine/shotgun/light/breaching, VENDOR_ITEM_REGULAR),
 		)
 
 /obj/structure/machinery/cm_vending/sorted/cargo_ammo/stock(obj/item/item_to_stock, mob/user)

code/game/machinery/vending/vendor_types/squad_prep/squad_prep.dm

@@ -279,6 +279,7 @@
 		list("M240 Incinerator Tank", round(scale * 3), /obj/item/ammo_magazine/flamer_tank, VENDOR_ITEM_REGULAR),
 		list("M56D Drum Magazine", round(scale * 2), /obj/item/ammo_magazine/m56d, VENDOR_ITEM_REGULAR),
 		list("M2C Box Magazine", round(scale * 2), /obj/item/ammo_magazine/m2c, VENDOR_ITEM_REGULAR),
+		list("Box of Breaching Shells (16g)", round(scale * 2), /obj/item/ammo_magazine/shotgun/light/breaching, VENDOR_ITEM_REGULAR),
 		list("HIRR Baton Slugs", round(scale * 6), /obj/item/explosive/grenade/slug/baton, VENDOR_ITEM_REGULAR),
 		list("M74 AGM-S Star Shell", round(scale * 4), /obj/item/explosive/grenade/high_explosive/airburst/starshell, VENDOR_ITEM_REGULAR),
 		list("M74 AGM-S Hornet Shell", round(scale * 4), /obj/item/explosive/grenade/high_explosive/airburst/hornet_shell, VENDOR_ITEM_REGULAR),

code/game/objects/items/stacks/sheets/sheet_types.dm

@@ -259,6 +259,8 @@ GLOBAL_LIST_INIT_TYPED(cardboard_recipes, /datum/stack_recipe, list ( \
 		new/datum/stack_recipe("empty magazine box (M41A Incen)", /obj/item/ammo_box/magazine/incen/empty), \
 		new/datum/stack_recipe("empty magazine box (M41A LE)", /obj/item/ammo_box/magazine/le/empty), \
 		null, \
+		new/datum/stack_recipe("empty magazine box (XM51)", /obj/item/ammo_box/magazine/xm51/empty), \
+		null, \
 		new/datum/stack_recipe("empty magazine box (M41A MK1)", /obj/item/ammo_box/magazine/mk1/empty), \
 		new/datum/stack_recipe("empty magazine box (M41A MK1 AP)", /obj/item/ammo_box/magazine/mk1/ap/empty), \
 		null, \
@@ -281,6 +283,7 @@ GLOBAL_LIST_INIT_TYPED(cardboard_recipes, /datum/stack_recipe, list ( \
 		new/datum/stack_recipe("empty shotgun shell box (Incendiary)", /obj/item/ammo_box/magazine/shotgun/incendiary/empty), \
 		new/datum/stack_recipe("empty shotgun shell box (Incendiary Buckshot)", /obj/item/ammo_box/magazine/shotgun/incendiarybuck/empty), \
 		new/datum/stack_recipe("empty shotgun shell box (Slugs)", /obj/item/ammo_box/magazine/shotgun/empty), \
+		new/datum/stack_recipe("empty shotgun shell box (16g) (Breaching)", /obj/item/ammo_box/magazine/shotgun/light/breaching/empty), \
 		null, \
 		new/datum/stack_recipe("empty 45-70 bullets box", /obj/item/ammo_box/magazine/lever_action/empty), \
 		new/datum/stack_recipe("empty 45-70 bullets box (Blanks)", /obj/item/ammo_box/magazine/lever_action/training/empty), \

code/game/objects/items/storage/belt.dm

@@ -903,6 +903,7 @@
 	var/drawSound = 'sound/weapons/gun_pistol_draw.ogg'
 	///Used to get flap overlay states as inserting a gun changes icon state.
 	var/base_icon
+	var/gun_has_gamemode_skin
 	can_hold = list(
 		/obj/item/weapon/gun/pistol,
 		/obj/item/ammo_magazine/pistol,
@@ -966,6 +967,14 @@
 		*/
 		playsound(src, drawSound, 7, TRUE)
 		var/image/gun_underlay = image(icon, current_gun.base_gun_icon)
+		if(gun_has_gamemode_skin)
+			switch(SSmapping.configs[GROUND_MAP].camouflage_type)
+				if("snow")
+					gun_underlay = image(icon, "s_" + current_gun.base_gun_icon)
+				if("desert")
+					gun_underlay = image(icon, "d_" + current_gun.base_gun_icon)
+				if("classic")
+					gun_underlay = image(icon, "c_" + current_gun.base_gun_icon)
 		gun_underlay.pixel_x = holster_slots[slot]["icon_x"]
 		gun_underlay.pixel_y = holster_slots[slot]["icon_y"]
 		gun_underlay.color = current_gun.color
@@ -1186,6 +1195,60 @@
 			"icon_x" = -11,
 			"icon_y" = -5))
 
+#define MAXIMUM_MAGAZINE_COUNT 2
+
+/obj/item/storage/belt/gun/xm51
+	name = "\improper M276 pattern XM51 holster rig"
+	desc = "The M276 is the standard load-bearing equipment of the USCM. It consists of a modular belt with various clips. This version is for the XM51 breaching scattergun, allowing easier storage of the weapon. It features pouches for storing two magazines along with extra shells."
+	icon_state = "xm51_holster"
+	has_gamemode_skin = TRUE
+	gun_has_gamemode_skin = TRUE
+	storage_slots = 8
+	max_w_class = 5
+	can_hold = list(
+		/obj/item/weapon/gun/rifle/xm51,
+		/obj/item/ammo_magazine/rifle/xm51,
+		/obj/item/ammo_magazine/handful,
+	)
+	holster_slots = list(
+		"1" = list(
+			"icon_x" = 10,
+			"icon_y" = -1))
+
+	//Keep a track of how many magazines are inside the belt.
+	var/magazines = 0
+
+/obj/item/storage/belt/gun/xm51/attackby(obj/item/item, mob/user)
+	if(istype(item, /obj/item/ammo_magazine/shotgun/light/breaching))
+		var/obj/item/ammo_magazine/shotgun/light/breaching/ammo_box = item
+		dump_ammo_to(ammo_box, user, ammo_box.transfer_handful_amount)
+	else
+		return ..()
+
+/obj/item/storage/belt/gun/xm51/can_be_inserted(obj/item/item, mob/user, stop_messages = FALSE)
+	. = ..()
+	if(magazines >= MAXIMUM_MAGAZINE_COUNT && istype(item, /obj/item/ammo_magazine/rifle/xm51))
+		if(!stop_messages)
+			to_chat(usr, SPAN_WARNING("[src] can't hold any more magazines."))
+		return FALSE
+
+/obj/item/storage/belt/gun/xm51/handle_item_insertion(obj/item/item, prevent_warning = FALSE, mob/user)
+	. = ..()
+	if(istype(item, /obj/item/ammo_magazine/rifle/xm51))
+		magazines++
+
+/obj/item/storage/belt/gun/xm51/remove_from_storage(obj/item/item as obj, atom/new_location)
+	. = ..()
+	if(istype(item, /obj/item/ammo_magazine/rifle/xm51))
+		magazines--
+
+//If a magazine disintegrates due to acid or something else while in the belt, remove it from the count.
+/obj/item/storage/belt/gun/xm51/on_stored_atom_del(atom/movable/item)
+	if(istype(item, /obj/item/ammo_magazine/rifle/xm51))
+		magazines--
+
+#undef MAXIMUM_MAGAZINE_COUNT
+
 /obj/item/storage/belt/gun/m44
 	name = "\improper M276 pattern M44 holster rig"
 	desc = "The M276 is the standard load-bearing equipment of the USCM. It consists of a modular belt with various clips. This version is for the M44 magnum revolver, along with six small pouches for speedloaders. It smells faintly of hay."

code/modules/clothing/suits/marine_armor.dm

@@ -78,6 +78,7 @@
 		/obj/item/device/motiondetector,
 		/obj/item/device/walkman,
 		/obj/item/storage/belt/gun/m39,
+		/obj/item/storage/belt/gun/xm51,
 	)
 	valid_accessory_slots = list(ACCESSORY_SLOT_MEDAL, ACCESSORY_SLOT_PONCHO)
 

code/modules/cm_marines/equipment/guncases.dm

@@ -348,6 +348,21 @@
 	new /obj/item/device/vulture_spotter_scope/skillless(src, WEAKREF(rifle))
 	new /obj/item/tool/screwdriver(src) // Spotter scope needs a screwdriver to disassemble
 
+/obj/item/storage/box/guncase/xm51
+	name = "\improper XM51 breaching scattergun case"
+	desc = "A gun case containing the XM51 Breaching Scattergun. Comes with two spare magazines, two spare shell boxes, an optional stock and a belt to holster the weapon."
+	storage_slots = 7
+	can_hold = list(/obj/item/weapon/gun/rifle/xm51, /obj/item/ammo_magazine/rifle/xm51, /obj/item/storage/belt/gun/xm51, /obj/item/attachable/stock/xm51)
+
+/obj/item/storage/box/guncase/xm51/fill_preset_inventory()
+	new /obj/item/attachable/stock/xm51(src)
+	new /obj/item/weapon/gun/rifle/xm51(src)
+	new /obj/item/ammo_magazine/rifle/xm51(src)
+	new /obj/item/ammo_magazine/rifle/xm51(src)
+	new /obj/item/ammo_magazine/shotgun/light/breaching(src)
+	new /obj/item/ammo_magazine/shotgun/light/breaching(src)
+	new /obj/item/storage/belt/gun/xm51(src)
+
 //Handgun case for Military police vendor three mag , a railflashligh and the handgun.
 
 //88 Mod 4 Combat Pistol

code/modules/projectiles/ammo_boxes/handful_boxes.dm

@@ -68,6 +68,20 @@
 /obj/item/ammo_box/magazine/shotgun/beanbag/empty
 	empty = TRUE
 
+
+//-----------------------16 GAUGE SHOTGUN SHELL BOXES-----------------------
+
+/obj/item/ammo_box/magazine/shotgun/light/breaching
+	name = "\improper 16-gauge shotgun shell box (Breaching x 120)"
+	icon_state = "base_breach"
+	overlay_content = "_breach"
+	magazine_type = /obj/item/ammo_magazine/shotgun/light/breaching
+	num_of_magazines = 120 //10 full mag reloads.
+	can_explode = FALSE
+
+/obj/item/ammo_box/magazine/shotgun/light/breaching/empty
+	empty = TRUE
+
 //-----------------------R4T Lever-action rifle handfuls box-----------------------
 
 /obj/item/ammo_box/magazine/lever_action

code/modules/projectiles/ammo_boxes/magazine_boxes.dm

@@ -170,6 +170,19 @@
 /obj/item/ammo_box/magazine/m4ra/heap/empty
 	empty = TRUE
 
+//-----------------------XM51 Breaching Scattergun Mag Box-----------------------
+
+/obj/item/ammo_box/magazine/xm51
+	name = "\improper magazine box (XM51 x 8)"
+	icon_state = "base_breach"
+	flags_equip_slot = SLOT_BACK
+	overlay_gun_type = "_xm51"
+	num_of_magazines = 8
+	magazine_type = /obj/item/ammo_magazine/rifle/xm51
+
+/obj/item/ammo_box/magazine/xm51/empty
+	empty = TRUE
+
 //-----------------------L42A Battle Rifle Mag Boxes-----------------------
 
 /obj/item/ammo_box/magazine/l42a

code/modules/projectiles/gun_attachables.dm

@@ -2105,6 +2105,43 @@ Defined in conflicts.dm of the #defines folder.
 	flags_attach_features = NO_FLAGS
 	hud_offset_mod = 2
 
+/obj/item/attachable/stock/xm51
+	name = "\improper XM51 stock"
+	desc = "A specialized stock designed for XM51 breaching shotguns. Helps the user absorb the recoil of the weapon while also reducing scatter. Integrated mechanisms inside the stock allow use of a devastating two-shot burst. This comes at a cost of the gun becoming too unwieldy to holster, worse handling and mobility."
+	icon_state = "xm51_stock"
+	attach_icon = "xm51_stock_a"
+	wield_delay_mod = WIELD_DELAY_FAST
+	hud_offset_mod = 3
+	melee_mod = 10
+
+/obj/item/attachable/stock/xm51/Initialize(mapload, ...)
+	. = ..()
+	select_gamemode_skin(type)
+	//it makes stuff much better when two-handed
+	accuracy_mod = HIT_ACCURACY_MULT_TIER_3
+	recoil_mod = -RECOIL_AMOUNT_TIER_4
+	scatter_mod = -SCATTER_AMOUNT_TIER_8
+	movement_onehanded_acc_penalty_mod = -MOVEMENT_ACCURACY_PENALTY_MULT_TIER_4
+	//and allows for burst-fire
+	burst_mod = BURST_AMOUNT_TIER_2
+	//but it makes stuff much worse when one handed
+	accuracy_unwielded_mod = -HIT_ACCURACY_MULT_TIER_5
+	recoil_unwielded_mod = RECOIL_AMOUNT_TIER_5
+	scatter_unwielded_mod = SCATTER_AMOUNT_TIER_6
+	//and makes you slower
+	aim_speed_mod = CONFIG_GET(number/slowdown_med)
+
+/obj/item/attachable/stock/xm51/select_gamemode_skin(expected_type, list/override_icon_state, list/override_protection)
+	. = ..()
+	var/new_attach_icon
+	switch(SSmapping.configs[GROUND_MAP].camouflage_type)
+		if("snow")
+			attach_icon = new_attach_icon ? new_attach_icon : "s_" + attach_icon
+		if("desert")
+			attach_icon = new_attach_icon ? new_attach_icon : "d_" + attach_icon
+		if("classic")
+			attach_icon = new_attach_icon ? new_attach_icon : "c_" + attach_icon
+
 /obj/item/attachable/stock/mod88
 	name = "\improper Mod 88 burst stock"
 	desc = "Increases the fire rate and burst amount on the Mod 88. Some versions act as a holster for the weapon when un-attached. This is a test item and should not be used in normal gameplay (yet)."