DML, you may have better info than I do, TTBOMK, the closest non 870 issued is at Quantico.
Local depts are unanimous about the 870, tho I understand B-more has some 37s in reserve. Some small local agencies may allow their officers to supply their own. Sometimes, I understand, they use confiscated weapons.
IMO, the 590 is at no great disadvantage to the 870 as an issue weapon, IF the dept rotates their inventory at regular intervals and has adequate armorers. Many do not. Same applies to many depts with 870s.
Nascar....
Politics do enter the awarding of dept contracts. That's why Md State Troopers tote the Beretta 92 on duty.They're made in Accokeek,Md. The troopers I know are not too enthused over the 92,many having retained the older Model 19 S&W that was the previous duty sidearm and carrying it as "Backup".
More often tho,the politics are over who gets to supply a given weapon,not which weapon.Dept heads are too litigation shy to go for many substandard weapons. Maintenance, training and so on are less subject to review,since any glaring shortfalls can be excused on the grounds of budget restrictions.
And, 500s and 1300s are good shotguns. I doubt either will last as long as an 870, but it takes a lot of shooting and neglect/abuse to make either inoperable. For personal rather than issue shotguns, the difference is probably it's your grandchildren rather than your great grandchildren that'll think Gramps's shotgun is getting a little loose.
I guess my lifetime sum of shooting 870s, both mine and issue arms, is around 50K rounds. I recall no glitches, except for a few deliberately induced and some when checking out defective weapons to see how defective they were.
But, if I had shot the same number of rounds through well maintained 500s or 1300s, I doubt the results would have been quite as good. 50K rounds is way more than most shotguns, other than target guns, ever see.I do not think the results would have been much worse.