As above, virtually ANY American pump or auto shotgun has been used by the military and police.
The military usually just issued as-needed contracts for shotguns and didn't "officially" adopt them.
From 1897 to the 1950's the Big Three were the Winchester 1897, Model 12, and the Ithaca Model 37.
Interestingly, all three were John Browning designs. The Model 37 was first made by Remington, then Ithaca got it.
Things changed in the 1960's in two regards.....
First, most makers could no longer afford to produce the old forged and milled steel "Tank" shotguns, so they began producing guns made of "Space Age" materials, i.e. aluminum, plastic, and stamped parts.
Winchester killed themselves in 1964 when they canceled their older guns and introduced the Model 1200.
Other makers introduced their own aluminum and stamped models.
Only Remington got it right with their Model 870, made from a forged steel receiver and with heavy duty fabricated internals.
The second big change came with the increased crime rate of the 60's and 70's.
Prior to this police shotguns were "riot" guns....... They sat in police racks until there was a riot, man hunt, or barricaded suspect.
Then the guns were issued and after the emergency were returned to the racks until the next time.
Under those conditions the old "tank" forged steel guns would last almost forever.
With the increase of violent crime and police shootings, departments began putting shotguns in every patrol car and the cops were encouraged to use them instead of the pistol.
The old steel guns eventually wore out and had to be replaced.
Under these conditions of shotguns used by everyone, but "owned" by no one, the guns got abusive treatment such as the night watch version of "Hold my beer and watch this".
The result was that the "Space Age" guns simply failed to stand up.
Winchester told police buyers that the Model 1200 was a great gun, better then the old Model 97 and 12, but the actual results were the guns just failed.
So, Winchester did a fast redesign and brought it out as the Model 1300.
It failed too, and after that virtually no large police agency ever bought a Winchester shotgun again.
S&W attempted to make a move on Remington with the Model 916.
It was such a disaster in both the police and commercial market that at one point S&W actually had internal discussions about recalling them all to protect the S&W name.
They made a second attempt with the Japanese Howa Model 3000, which was a Remington 870 clone.
For a short time it had a cult following among individual police officers, but the gun had nothing to recommend it over the Remington 870, so due to lack of sales S&W dropped it.
Mossberg picked it up as the Model 3000 for a few years, then also dropped it.
Mossberg made their try with the Model 500.
Only a large Tennessee agency bought it and soon dropped it after it too failed to stand up.
By this time US law enforcement had settled on the Remington 870 because of it's proven ability to stand up to police use.
Virtually every law enforcement agency; local police, sheriff, State police, and Federal used and mostly still use the 870. Until the last 10 years or so, Remington held 95% plus of the LE market.
Currently other makers are starting to break in to the military and police market with new designs.
Mossberg having failed with the 500 introduced a much beefed up 500 gun as the Model 590 and 590-A1.
They won a contract to supply 590-A1 shotguns to the US Government.
Their advertising bragged that "Only the Mossberg was able to pass the grueling government firing test and win the contract".
The truth is ONLY Mossberg entered a shotgun in what was actually a pass-fail test.
The test was not a "What gun's the best" it was only to pass the fairly easy to pass firing test.
If a gun could pass the fairly easy test, the lowest bidder would get the contract.
Since companies like Remington could not possibly make a forged steel receiver gun as cheaply as Mossberg's extruded aluminum gun, no other maker bothered to enter a gun for the test.
In recent years a number of newer guns using modern manufacturing methods have begun to be able to stand up to military and police use so they are starting to make inroads in the market.
Through out the history of the 20th Century, whenever the military or law enforcement needed shotguns, most any available gun would do so virtually all of them have seen service at one time or another.