[1] While shotguns had been used in earlier conflicts, the trench warfare of World War I demonstrated a need for standardized weapons and ammunition.
These cartridges became wet in the muddy trench warfare environment; and swelled paper cases would no longer chamber reliably.
Full-length brass cartridges proved more resistant to moist field conditions and the repeated loading and unloading during patrols and watches when no ammunition was fired.
Heavy bullet preferences dating back to the Philippine–American War were re-evaluated considering combat experience with the 5.56×45mm NATO cartridge, and advantages of a larger number of smaller No.
[1] Plastic cases loaded with small steel darts called "flechettes" were issued on a limited trial basis during the Vietnam war.
[4] Flechettes had flatter trajectory over longer ranges than spherical buckshot, but combat effectiveness did not justify continued production.