55gr FMJ BT–which is pretty much the most common .223 ammunition–tends to fragment pretty dramatically inside about 300y after hitting something solid. Once it hits something solid, it starts tumbling and tears itself apart pretty fast, in part because it’s moving at supersonic speeds. That’s part of the reason why it’s so lethal at short ranges; it’s turning into a lot of small fragments… (That, and the cavitation that is produced by bullets moving >2600fps; that will produce a temporary wound cavity that exceeds the elasticity of tissue, and turns into a permanent wound cavity. The cavitation produced by subsonic bullets isn’t great enough to turn into a permanent wound channel.) At subsonic speeds, it doesn’t fragment, and just ‘ice picks’. A shotgun with birdshot would definitely not over penetrate, but then you run the risk of not being adequately lethal on your target. Buckshot is still going to penetrate exterior walls pretty handily without being deformed significantly or fragmenting, even if a lot of the energy has been eaten up by a house’s cladding; it might not kill easily, but it can still wound. But then you’re back to the original problem: large/long firearm, heavy recoil, very limited ammunition in a tube magazine, and slow to reload.
55gr FMJ BT–which is pretty much the most common .223 ammunition–tends to fragment pretty dramatically inside about 300y after hitting something solid. Once it hits something solid, it starts tumbling and tears itself apart pretty fast, in part because it’s moving at supersonic speeds. That’s part of the reason why it’s so lethal at short ranges; it’s turning into a lot of small fragments… (That, and the cavitation that is produced by bullets moving >2600fps; that will produce a temporary wound cavity that exceeds the elasticity of tissue, and turns into a permanent wound cavity. The cavitation produced by subsonic bullets isn’t great enough to turn into a permanent wound channel.) At subsonic speeds, it doesn’t fragment, and just ‘ice picks’. A shotgun with birdshot would definitely not over penetrate, but then you run the risk of not being adequately lethal on your target. Buckshot is still going to penetrate exterior walls pretty handily without being deformed significantly or fragmenting, even if a lot of the energy has been eaten up by a house’s cladding; it might not kill easily, but it can still wound. But then you’re back to the original problem: large/long firearm, heavy recoil, very limited ammunition in a tube magazine, and slow to reload.