Thoughts On .Mil Breaching Shotguns:
The shotgun is a special purpose weapon, not a primary one. It is intended for use to ballistically breach locks/chains/hinges and other targets as needed in order to breach and secure entry. That being said, it is also considered a secondary weapon to kill bad guys with and to transition to, especially if a handgun is not carried.
I believe that the shotgun is a fighting shotgun first - as such it needs sights and a light. A dedicated breaching light is also Value Added to minimize visual signature prior to breach. VIS Red / IR to illuminate target and structure is my preferred option.
Ballistic breaching may be a better option if the Unit is not trained in explosive breaching, or has limitations using that operationally. (In OEF we ended up doing a lot of covert entry because it did not damage villager's locks/doors/gates.) This should clearly be delineated in a Breaching use of Force Continuum.
I am not a fan of standoff devices on .mil shotguns due to the need to move distances on foot and over terrain features. Proper technique can be used without the aid of a standoff device and standoff devices, even when properly secured in a weapons catch or sheath, can tear you up.
Status: Shotguns are carried with 5+1 in chamber. Weapon is on safe. The M500 safety is superior to the 870 based on location and ease of use, and also is difficult to inadvertently pace on fire.
Shotgun Carriage: I do not advocate shotgun carriage being on your back. This involves needing to "fish" for it with one hand over your back, has a tendency to flag you and others, and typically requires 2 PAX to resecure. Shotguns make a large hole on entry. While I still like the old Eagle Industries Shotgun Scabbard and cross draw, lightweight magnets for retention are the new standard. Previously this option had a large weight penalty, today this is no longer the case. A single point sling and weapons catch can also work well, though both are needed to function efficiently. Without a weapons catch, the single point sling will wave the shotgun all over, especially when climbing and traversing obstacles.
Ammunition: The issued Winchester 00 Buckshot round has terrible pattern density and spread. I also do not advocate it to be used for ballistic breaching due to increased risk of causing injury to self, fellow teammates, and unknown's inside. There are dedicated frangible breaching rounds in the system, however actual procurement of them can be difficult. Combined with not being authorized COTS ammunition to use, this becomes problematic and is a major reason why ballistic breaching in the GPF is not in common use (other factors being lack of training, lack of schools for ballistic breaching, lack of budget, training materiels, command influence.)
Ammunition Carriage: I've been searching for this Holy Grail since 2007. The best option I have found to date includes industrial-strength pile Velcro on the shotgun itself and hook Velcro shotgun cards. RCS/Ares and Esstac Shotgun Cards are my preferred option there. These can also be placed in single magazine pouches and shingles. Just like magazines, shotgun shell cards are disposable items that require PMCS. Constant loading/unloading will loosen elastic. The new QD-C solution intrigues me as well, I'll be testing that when it is released commercially.
TTP's: Not going in depth here. .Mil tends to see less traditional targets: a lot of gates, locks, chains that require breaching. Metal vs. Wood have different attack methods.
Barrel Length: Shorter barrels are preferred; I've had .mil shotguns with 18.5", 16.5",14", 12.5" barrels. I prefer 5+1 Capacity, but I'll go down to 3+1 if there is another method of breaching available if the ballistic breach fails.
M500 Force Mod: Rail on pump for attachment of lights. Rail on top for optics (RMR, low mount T1). In OIF I was using zipties on an E2D and an LED keychain light Velcro'd to the shottie. There are better, purpose built solutions and means of attaching them now thankfully.
The M26 is absolute garbage. Weapons mounted to M4's are not a very good idea on principle (barrels not free floated, you are incurring a weight and footprint penalty, ranges do not support use of 40mm and 5.56 to allow Grenadiers to choose which weapon is more suited to the task at hand). The use and operation of the M26 itself is not conducive for Infantry use - it is not semi-automatic and requires straight pull bolt action. This makes incredibly slow for multiple shots on targets, especially when mated to the M4. The box magazines are 3 or 5 round, which is less capacity than the M500. It requires you to have more pouches on your kit to store them, and you cannot consolidate or manage ammunition (partial and empty magazines) as effectively. Gripping it is entirely non-intuitive and doesn't aid in recoil management for Push/Pull technique. This image alone says it all:
We also had the opportunity to test the Crye Six12 and same issues I identified with it without seeing it in person manifested themselves live. It's just not suitable for a .mil breaching shotgun.
The KSG-12 performed well, although I have reservations about Kel-Tec's track record, materials used in the KSG-12, long-term durability and reliability. lack of commercially available spare parts, and no armorer courses or support without sending it back to Kel-Tec.