I may have heard a few things about the issue shotguns over the years:
TC’ed and procured in the 1970s qualified under MIL-S-3443 thru rev E:
Type I. With bayonet adaptor, heat shield and sling, (20" M-590)
Type II. With sling only, (17/18 " 590A1/M-500/500MLS Primary Issue Shotgun for DoD)
Type III. With rifle sights and sling
Original 1970’s Procurement Objective:
MIL-S-3443: 6.1 Intended use. The shotguns furnished to this specification are intended for military guard duty, riot control and combat.
Modern Employment of the Shotgun:
Shotguns are primaryly used as secondary weapons/tools for employment of less lethal or breaching munitions. Shotguns today are employing munitions not originally envisioned under MIL-S-3443 such has the M-1012, M-1013, and M-1030 Breaching Round.
The addition of a pistol grip and improved cleaning kit are the only systemic modernizations in over 30+ years of service.
Shotguns are in high demand in GWAT and are frequently modified by combatant commanders with commercial add-ons .
Army:
Mossberg 500 NSN 1005-01-295-1832, 18 1/2", brass bead sight with over pack pistol grip. Commanders use pistol grip or standard issue stock as required.
Navy/USMC:
Mossberg 590A1: NSN 1005-01-295-4482 17" barrel, issued with standard full stock
Very small numbers of the 20" 590A1's appoximately 1,000 were issued mostly to SOF and MP units.
Most 14" Mossbergs were issued under forign military sales. For example the Afghan Army and Police have alot of 14" Mossbergs.
Appoximatly 30,000 Mossbergs are in service with DOD, 10,000 M-1014 Benellis and a smaller number of 870's. Most (but not all) 870s are being retired as they were bought a LONG time ago read before 1980's. Any remaining Winchester were to have been turned in for replacement with Mossbergs several years ago for at least the Army.
There have been some recent official modernization efforts.