Ammo box explosion caps at 3k bullets (#8389)

# About the pull request
Ammo box explosions max out at 3k bullets, previously you can toss an
entire box of m56 and create an earth-shattering kaboom. Now the
explosions are caped at 3k bullets.

Previously: M56D box detonated center of briefing (5.6k bullets)

![image](https://github.com/user-attachments/assets/ebd74d66-2317-4fb2-b0f6-54c4a92e6dc8)

After 3k bullet cap:


![image](https://github.com/user-attachments/assets/6b457e34-1f22-4dd6-90ee-edfc6a3f1731)

Did some testing, does 350~ damage to adjacent xenos, enough to kill
base runners but leave acid runners alive on a sliver of health (and
stunned).
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# Explain why it's good for the game
Players shouldn't be able to create explosions that big just by filling
up a few m56d boxes from their preps (which nobody takes anyway).
# Testing Photographs and Procedure
see above


# Changelog
:cl:
balance: Ammo box explosions are capped at 3k bullets.
/:cl:

code/modules/projectiles/ammo_boxes/ammo_boxes.dm

@@ -220,7 +220,7 @@
 	else
 		for(var/obj/item/ammo_magazine/AM in contents)
 			severity += AM.current_rounds
-		severity = floor(severity / 150)
+		severity = clamp(severity / 150, 0, 20) // explosion caps at 3k bullets
 	return severity
 
 /obj/item/ammo_box/magazine/process_burning(datum/cause_data/flame_cause_data)