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“...Departments started putting patrol rifles in cars for exactly the opposite reason that these reporters are claiming...departmental risk management groups came to realize that a 12 gauge is just “too much gun” for a lot of law enforcement problems.

The solution? The relatively more accurate, and much less powerful AR, usually chambered in the civilian .223 cartridge. Much less risk of over-penetration compared to a 12 gauge slug or even buckshot, and zero “spread” compared to shot...” [absalom01, post 31]

Exactly opposite of actual terminal ballistics.

5.56x45mm - 223 Remington in civilian loadings - far outranges any shotgun, any loading. Slugs have the edge in momentum but total energy is comparable. Rifle bullets always penetrate deeper than shot charges - in any medium.

There is the question of “stopping power” - a topic that preoccupies many enthusiasts but defies quantification and predictability. The Defense Dept does not use the attribute in cartridge selection, and law enforcement agencies follow their lead.

All of which leaves the question unanswered: why would a police dept turn in shotguns for MSRs?

Best guesses: controllability, magazine capacity, ease of reloading.

Most shotguns suitable for law enforcement use have tubular magazines. Slightly fussy to load, one loose round at a time. Even with extended magazine tubes, max capacity rarely exceeds nine rounds.

Shotguns develop serious recoil with full-power loads. Even the best users can find this off-putting; hampers response in a real-life emergency situation. This may be changing as ammunition manufacturers offer more varieties of reduced loads and shorter shells (the latter increases magazine capacity - if the shorties can be induced to feed right).

By way of contrast, autoloading rifles chambered for an “intermediate” cartridge feeding from detachable box magazines develop gentler recoil, hold many more rounds, and reload more easily & speedily. Various platforms fit the bill but AR-15-style rifles satisfy requirements neatly; and there are added advantages - three generations of military development, potential federal grants, parts commonality with military organizations, and possible resupply.

Few law enforcement agencies are administered by arms experts. The bureaucrats and politicians who command them know even less; other priorities likely loom larger. The reasons “why” may never be revealed.