Yeti (Steve Fisher) knows more about them than I and has a larger data pool.
I've been doing a T&E of the KSG-12 for awhile now, but I am only a sample of one with a personally owned weapon system. I am not a fan of bullpups due to difficulty in loading as well as my face being closer to a potential kaboom, but it is a solution applicable for the shotgun.
Installation, Operation, Maintenance (IOM)
NEGATIVES:
1) Poor Materials / Composition For A Firearm
Plastics are not an ideal material for a firearm as they are not as reliable / durable as others.
2) Decreased Life Cycle
With the subpar materials used, life cycle of the weapon system and parts is decreased. Another problem is that unlike the M4 FOW there is not a standard out for periodic parts replacement at round count intervals.
3) No Spare Parts Commercially Available
There are a lack of spare parts commercially available. There are a few upgrades to parts that are COTS - but no dedicated spare parts or replacement packages.
4) No Armorer Support
This and #3 are mainly what killed our looking into the KSG-12 for adoption as a ballistic breaching platform. There are no Armorer Courses or Armorer Support for the platform. The weapon system must be sent back to Kel-Tec which would leave the Ballistic Breacher without a materiel solution to accomplish his mission (or he'd revert to the M500, but training time is limited for initial & sustainment training to build proficiency.)
POSITIVES:
1) Ammunition Capacity:
The KSG-12 has 14+1 capacity for 12 Gauge Shotgun Shells. This allows for 15 shotgun shells being expended before you need to reload.
2) Internal Ammunition Storage;
The KSG-12 doesn't have large obtrusive magazines such as those in rival 12 gauge platforms, to include the Saiga 12. The issue with magazine-fed shotguns is where to store additional magazines, where to store partial magazines, and how to consolidate ammunition.
3) Modularity:
The KSG-12 has an aluminum top rail for attachment of SOPMOD accessories such as optics, lights, lasers. It has a shorter bottom rail as well.
4) Length:
Overall Length (OAL) is approximately the same as my Mossberg M500 in ballistic breaching configuration (pistol grip).
5) Safety: Safety is similar to M249/M240/MK48 weapon safeties. It is easy and intuitive to use and compliments the existing manual of arms. My principle is engaging the safety during reloading and prior to movement - this compliments it.
Operation (IOM):
Ammunition Capacity is 14+1 12 Gauge Shells. Two tubes hold 7 shells each. You can select via lever which tube will utilized / able to be loaded. The issue selector is small, there are larger aftermarket selectors available. Only one tube can be loaded at a time.
Loading: Loading the KSG-12 is difficult and time-consuming. Initially I turned the weapon system upside down to load during initial train-up and for Yeti's course. I've since transitioned to simply canting the weapon system 90 degrees pointing it downwards and going by feel. The reason I did this was because I needed a reloading system that would work in light, low light, no light, et al. If I have to load support hand I cant it 90 degrees the opposite direction.
Invest in some inert 12 Gauge training rounds if you truly want to become proficient in reloading, which you should aspire to be.
Action: The action is not as smooth as the Remington 870 or Mossberg 500. I had a bit over 5% failure rate where I negligently short stroked the KSG-12 pump action. More training time to include dry practice and a Magpul grip stop have reduced this.
Loading: The KSG-12 is more amenable to loading via the Louis Awerbuck method of loading the tube first and then chambering a shell followed by feeding more shells into the tube. This makes Slug Select Drills much more difficult and at risk of higher rates of malfunctions. That is not an issue with me as I only run buckshot in my shotguns.
Malfunction Clearance: Immediate Action is easy to conduct. Remedial Action is more difficult compared to the Remington 870 & Mossberg M500. This is mostly due to difficulty in extracting shells by hand.
Modularity: The KSG-12 offers decent modularity, though not as much as the M4 FOW. As issued, there is a top rail and and bottom rail.
I have a front iron sight, breaching light(s), white / IR light, IR/VIS Laser, RDS, Magnifier, rear iron sight on mine. I (jokingly) tell people it's the weapon I'd risk UCMJ action to bring overseas with me.
Mesa Tactical makes an aftermarket rail for them. Their regular rail is better (and has two attachment points on the side for Picatinny sections), the Sidesaddle Rail sticks out too far and negatively affects weapon weight and balance. I have breaching lights set up on the smaller Picatinny sections.
I recommend against QD mounts on the KSG-12 due to the increased recoil impulse which will toss accessories off it like a bull. Loctite is your ally.
The bottom Picatinny rail is plastic, not aluminum. As such, it is not as effective as an aluminum rail and will have increased rate of equipment loss due to this and recoil impulse.
I have a kydex cheek piece that fits over the stock which has Velcro on it for an Esstac shotgun card. The card can be pulled off when empty and replaced with another.
My other kydex cheek piece has that and a 3/16" circular recoil pad on it. They can be popped off and replaced easily.
Maintenance (IOM):
As stated, -10 Level Maintenance is practically nonexistent unless you need to replace issue parts with parts that available aftermarket.
No spare parts commercially available, no Armorer Courses, no Armorer Support. Weapon systems must be sent back to Kel-Tec for repair.
SPECIALTY ROLES:
In addition to ballistic breaching, I believe the KSG-12 has merit in the role of Less Lethal.
Every time I watched footage of Ferguson PD responding to the riots I wished those studs let loose with KSG-12's loaded with Less Lethal rounds with interlocking sectors of fire. I would pay good money to see that converging effect on a crowd.
I'll publish photos of my current iteration after my work contract is complete (I have an article about it in the works but can't publish due to terms) - but that'll likely be awhile.
I'll probably buy another just to have a spare weapon system on hand.