So I went down a wiki hole after looking over this and the following post on Forgotten Weapons. That made me look up SALVO. They thought they might be onto something with a flechette weapon that on paper fire 2300 round per minute and was controllable even though it only weighed three and half pounds, loaded with 60 cartridges. Can you imagine? If that had been more workable we'd have never even heard of the AR, maybe. AR would be a footnote like a Johnson rifle. Their are PISTOLS that weigh more than 3.5 pounds.
Doctrine would involve a lot more Spray and Pray suppressing fire, I'm thinking. There was also a thought, made new again recently, of adding a 12 gauge style grenade launcher. A smaller M203. Like a XM25, but 50 years ago. So, picture Viet Nam with a even MORE walls of bullets and lots of booms from the grenades of every squad member. BrrrrrraaaaaapppppBOOM! BrrrrrraaaaaapppppBOOM! BrrrrrraaaaaapppppBOOM!
But, as we know, such a future-gun didn't really make it out of the gate.
The search for a solution that was solved by the .276 Pedersen was instead settled by the .223. After overcoming LOTS of inertia, prejudice for stopping power over volume of fire, and a conservative mindset in the people in the Army in charge of ordnance decisions. The opposing forces still hanker for more stopping power. Imagine if we had adopted the .276 pre-WWII. We'd probably have people today looking to lessen the ammo weight and increase volume of fire with something like the .223. And the wheel turns and turns.
"You never select a shotgun as your primary anti-zombie firearm. It's great for onesy twosey, but zombies travel in hordes. The reload time is onerous, and the ammo, while effective, is heavy and bulky and short ranged."
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