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Updated 22-3-10
Shotguns.

         Shotguns are useful when there is a desire for close range stopping power or high hit probability. They are also used when there is a need for intimidation or the application of specialized ammunition.

        The Shotgun has been in military use since at least the First World War. (One of the novel uses it was put to in this conflict was that Skeet shooters were used to fire at rifle grenades.)
        The military shotgun has changed very little since this time. Of course, if something can do the job effectively, there is no point in changing it just for change's sake, but the fact remains that the tube-magazined pump action shotgun is not the most effective configuration for a combat weapon.
        The claim that only pump actions are reliable enough for military service is refruted by the fact that since the 50's the standard British army weapon was the recoil-operated Browning Five, which has been used in such punishing environments as the Malayan jungle.

        A more objectionable feature is the tube magazine, which the repeating shotgun seems to have inherited from the Winchester rifle. A practical shotgun shooter once explained to me that the shotgun shell is in fact one of the worst proportioned rounds to use in a tube-mag. Even if this is not true, the tube mag is a slow and sometimes fiddily device to reload.

        In 1999 the US adopted the Joint Services Combat Shotgun, but disappointingly failed to use this opportunity to adopt a design with any improved features. In fact, the weapon adopted is a version that is semi-automatic without a pump option, making it less effective if using low impulse rounds. Beneli do actually offer this weapon in a version that can be converted from pump to self-loading mode with just a twist of a control, but this was not adopted.

        Shotgun powder is fast-burning and a 12 bore barrel has lots of volume. A shotgun propellant charge will be consumed in less than 18” of barrel. According to Remington modern powder in a shotgun burns completely in 25 to 36 cm (10-14”) of barrel. This article offers the following interesting information:-

        “.....decreasing the length of barrel by 26 %, (from c23” to c17”) the velocity loss is only 6.75 %, while the energy loss equals 6.25 % (shell size 12/70). Corresponding loss of values of the above parameters in the case of 12/73 mm shells equals 7.28 % and 8.0 %.”

        An independent test on the Saiga Forum had similar findings:-
Birdshot- 12 gauge, 2 ¾ inches, 1 oz shot
12”     barrelaverage 1040 fps
14”     barrelaverage 1069 fps
18”     barrelaverage 1114 fps
19½”  barrelaverage 1114 fps
20”     barrelaverage 1108 fps
28”     barrelaverage 1159 fps
Remington 00 Buck
12”     barrelaverage 1121 fps
14”     barrelaverage 1127 fps
18”     barrelaverage 1216 fps
19½”  barrelaverage 1205 fps
20”     barrelaverage 1182 fps
28”     barrelaverage 1250 fps

        Reducing barrel length 16” from 28” to 12” reduced velocity only 10% and only 119-129fps, less than the variation in factory loaded ammo in many boxes of shotshells. Change from 20” to 12” is only 7%.
        A common misconception is that shortening a shotgun's barrel will increase its spread. Other than the effect that removing the choke section has experiments have shown that shortening the barrel has very little effect on spread until you get really short. One set of such experiments was reported in the February 2008 edition of Small Arms Review Magazine. A shotgun barrel was progressively cut down and the spread of shot at 15 yards recorded. With a 30” barrel spread was 10”, with a 24” barrel about 11”, with an 18” barrel 8” spread and with a 12” barrel 12½” spread. These variations in pattern diameter are probably not significant. Only when the barrel was cut down to 6-7/8” did the pattern open out to 17”. Other experiments that I have heard about seem to have similar findings.
        Shotguns are used for close range combat and often used in cramped conditions. A combat shotgun does not need a long barrel. A combat shotgun probably doesn't need a barrel of more than 12-14” length. Another objectionable feature of the tube-magazine is that it necessitates that the weapon has a long barrel if the weapon is to have a useful ammunition capacity.

“Both the SOW (Special Operations Weapon) and its magazine-fed Remington 870 predecessor (bottom) were products of mechanical wizard Carroll Childers, an engineer at the Naval Special Weapons Center. The 870 mod kit provided SEAL shotgunners with a quick-change magazine holding 20 rounds. The SOW was full-auto.”
The Firearms Blog
        During the Vietnam war Carroll Childers produced a box magazine feed shotgun that I think was based on the Remington 870. Childers also produced 50rd magazines for the M16 and the Childers-Dahlgren –a tripod mounted blow-forward automatic shotgun with a rate of fire of 200rpm.
        Since then, several other companies have produced shotguns with box-magazines. Box-magazine shotguns work, and offer numerous advantages.
         The reluctance to accept these designs is endangering servicemen's lives.

        Other nations have not been so conservative. The French Navy has adopted a box-fed design. Shotguns are, of course, a very useful weapon for boarding operations, such as searches for contraband. Russian forces make use of the Saiga designs, and South Africa has a long tradition of innovative designs

Striker
Mag 7
Neostead

        I see two forms of shotgun answering most military needs.

Assault Shotgun
        The first one might term an “Assault Shotgun”, and is used in situations where close combat is highly likely. This is a good weapon for the pointman or scout of a jungle patrol. Encounters are often at very close range, and a couple of blasts from a shotgun are more likely to prove decisive than a burst from an SMG or Assault rifle.
        This weapon should be a self-loading or selective fire weapon with a box magazine. A manual override (pump action) for firing special rounds would be useful, however.
        There are several designs of box loading shotgun offered by several nations. The Franchi SPAS-15 is probably the most well know, but there are also more conventional looking weapons such as the Bernardelli B4. The Russian company of Izhmash offer a family of shotguns with either tube or box magazines, which would simplify logistics if an underbarrel weapon with a tube mag is also used. Izhmash also offer a model that has both a box and tube magazine.
        A nice example of a self-loading box magazine shotgun is the Russian Saiga 12K, based on the Kalashnikov. So too is the AA-12. Both designs would be more useful with a manual overide option. A vertical foregrip would be a good feature to combine with this

        The Daewoo USAS-12 shotgun offered the option of a drum magazine, and had a carrying handle compatible with M16 accessories. A weapon with similar controls to the M16, such as the safety would have obvious advantages.
        The weapons should have a barrel of about 12” and the muzzle should be threaded section of barrel so various accessories can be fitted. These devices can also be fitted to the muzzle of the Ripley (see later).

Underbarrel and Covert Shotgun.
        In MOUT a shotgun proves a useful means for remotely destroying door hinges and locks and triggering booby traps. Destroying locks and hinges often requires special ammunition (Hatton rounds) and multiple hits at close range, so other devices such as the Doorbuster may be more effective. If a shotgun is desired I suggest a compact shotgun that can be mounted beneath a rifle's barrel by a Picantinny rail or the interbar used for the product-improved M203. An underbarrel shotgun also offers the Soldier either lethal or less-lethal options for dealing with a target.
        An idea of what this might look like could be gained from the M41A pulse rifles used in the film “Aliens”. The “30mm grenade launcher” appears in reality to be a cut down Franchi SPAS-12. We'll call this underbarrel shotgun the “Ripley” in honour of the main character of the Alien movies.
        Underbarrel shotguns are not a new idea, but most companies that offer kits base them on pump-actions. The convertible version of the Beneli M3-T would make a near ideal basis for the Ripley.
        Despite my comments above, the Ripley would have a tube magazine to give the overall weapon a smoother configuration. Since the slide action may be a bit of a stretch to reach when the weapon is mounted on a rifle, I suggest a semi-automatic action with pump override. Another reason for the tube magazine and semi-auto action will be covered in a moment.
        The most likely rifle that the Ripley would be mounted on is the short barreled M4. This will keep to total package compact enough to use in confined spaces such as corridors, and keep the total weight down. Having an assault rifle available partially overcomes the slow reloading of the tube-magazine and allows the user to still defend himself at long range.
        The Ripley could also be used detached from the rifle. Such a weapon would be useful for special operations. In this role the Ripley is fitted with pistol grip and possibly a folding or telescopic stock. An M16 grip can be used though a plow-handle grip offers better control during rapid fire. Possibly an attachment point that will accept either could be used.
        In this configuration the Ripley offers itself as a useful close-range weapon for unconventional warfare. The tube magazine gives a smooth shape that can easily be concealed under a jacket or in a brief case. The semi-automatic option allows repeat shots to be made while the firer's other hand is occupied with controlling a vehicle or moving a principal or wounded comrade.

http://izhevsk.club.guns.ru/eng/symposium.html
USAS-12
Saiga 12
Ammunition.
        The two main antipersonnel rounds used with military shotguns are buckshot or flechettes that resemble finned nails. Buckshot has a proven record of effectiveness, but is short ranged and is unlikely to be effective against body armour. Flechette projectiles are reported to have good long range and body armour penetration, but I have seen doubts about their short range characteristics and terminal effects.

1” flechettes from Sabot Designs

        A better round might use both flechettes and pellets in the same cartridge. The flechettes would resemble arrowheads or paper aeroplanes stamped from metal, and with sharpened leading edges. These might be mass produced in much the same way as disposable razor blades. The pellets would be a drag stabilized design -either of a teardrop or shuttlecock shape with a nylon skirt.
        The two types of projectile are designed to be complimentary in shape, so they can be nested together and a large number fitted in the same cartridge. Both types of projectile will have better penetration than standard buckshot. The flechettes give long range capability and the sharpened edges will be effective against soft body armour. The pellets give close range knock down effects and are useful for shattering locks and hinges.
        These rounds would be complimented by a HESH or HE slug with enhanced incendiary effects. This would have an impact fuse but also have a timed self-destruct. This allows the round to air-burst when it reaches the limit of its effective range or if it embeds in soft material such as sandbags.

Gelatin testing of Shot loads.

Frag-12 Explosive Shotgun Rounds
More on the Frag-12 (Scroll down)

        All three FRAG-12 rounds utilize a standard 3-inch 12-gauge cartridge case and propellant, which fires a fin-stabilized 19mm warhead with a MIL-SPEC 1316-compliant fuze assembly. The projectile arms after firing once it reaches 3 meters from the muzzle, and detonates on impact with the target.
        There are three members of the FRAG-12 munition family: the FRAG-12 High Explosive (HE), FRAG-12 High Explosive Fragmenting Antipersonnel (HEFA, or HE-FA), and FRAG-12 High Explosive Armor-Piercing (HEAP, or HE-AP).
        Initial testing has confirmed that the HE Blast round will produce about a 1- inch hole in cold-rolled steel plate with secondary spalling effects on the downrange side of the plate. The HE Fragmentation warhead containing “90 stainless steel ball bearings” (BB shot?) is designed to have blast and fragmentation out to a 2-meter casualty radius. The HEAP incorporates a shaped charge and is claimed to be able to penetrate 4 inches of aluminum armor and more than ½ inch of steel.
         All three rounds have a 200m effective range.

         To Quote the Defense Review Article:-
        To give the reader an idea of how just one FRAG-12 round can perform against a non-armored vehicle (standard car or truck/SUV), here’s what Randy Aukamp of Action Manufacturing (FRAG-12 subcontractor) told Defense Review about a test they conducted with a single HE-FA anti-personnel round:
        “We fired the [FRAG-12 HE-FA] fragmentation round into a car filled with probably 60 balloons packed in there. The round penetrated the front windshield just below the rear-view mirror, went probably 2-3 inches past the windshield, detonated, and took out the driver and passenger side windows, and took out all the balloons at the same time. The back window was also blown out, and the rear side windows were pelletized up.”
        Basically, the round saturated the interior of the car with its 90 ball bearings. “Nothing in that car would have survived”, Cates said. And that’s just one round.


Reduced Recoil and Reduced Length Shotshells.
        Combat shotgun rounds don’t generally need the range of hunting rounds so by reducing the propellant charge or shotload (or both) a 12 bore round can produce less recoil and allow quicker follow-up shots. This also simplifies training. Some Reduced-Recoil loadings also offer reduced muzzle flash, a useful feature in low-light conditions or when using light amplification equipment. Be aware, however, that many of these reduced recoil rounds may not have sufficient power to cycle an automatic shotgun. One exception is the Hornady TAP Light Magnum loads which are designed to produce reduced-recoil and still work in semi-autos.

        If you are reducing the shot load and/or propellant then you don’t really need a 3” shell case, or even a 2¾”. Aguila have taken this line of thought to the logical conclusion and produced a range of 1¾”(44mm) shotshells for police and home defence use. One loading contains six No.1 (0.30”) pellets, another a dual load of seven No.4 and four No.1 shot. There is also a slug load and a birdshot load with around 220 No. 7½ pellets. The advantage of the Minishells is that being a third shorter than standard rounds you can fit up to 25% more inside a tube magazine. Downside is that some guns will need a modified shell elevator to use Minishells, although the modification still allows the use of conventional ammo. ALS produce 2” (48mm) shotshells containing loads such as 27 No.4 (0.24”) pellets. These are reported to function in standard slide actions. Neither the ALS 2” or Aguila Minishells appear to be recommended for use in self-loaders. A shortened round that will cycle in a self-loader should be possible.


Quadrangle Buckshot and Slug.
        MK Ballistic Systems offer a type of shotload they call QB-8 Quadrangle Buckshot. This resembles a hardened steel cylinder that has been divided into eight right-angle wedges. Each wedge has six pointed corners, seven sharp edges and five flat surfaces and the round is claimed to be capable of defeating NIJ level IIA body armour. The wedges are held in a discarding plastic sabot to protect the shotgun’s bore from damage. The lighter weight of steel and the poor ballistic shape of the wedges limits effective range to 25m.

        Also offered by the same company is QB-Slug. Again, the projectile is a steel cylinder divided into eight wedges, but in this round the plastic sabot holds the projectile in one piece until after it has impacted with its target. The QB-Slug is designed as an anti-vehicle and anti-material round for use automobiles, light aircraft and boats. The break-up of the projectile into multiple sharp edged fragments increases the chance of components such as wiring being damaged.

UPDATE
        Another consideration for future designs is the adoption of some form of OICW. A canister round for this weapon would make alot of sense, and it is logical that the same round could be used in a Ripley type weapon

Further UPDATE
        Troops in Afghanistan being issued with box-fed underbarrel shotgun. This is a manually operated weapon cycled by a handle on the left side. While the box mag is a step forward needing to manually cycle each round by bolt-action is not.

Box-feed Underbarrel Shotgun.
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