"Duplex" loads grew out of the shotgun's past a long way back. The old "buck and ball" Revolutionary war era loads in smoothbore muskets were the original predecessors, as far as I know.
The British used shotguns a lot during the insurgency in Malaya in the early 1950s (where the 870 first saw combat), and developed a duplex load for their Browning and Remington shotguns. Their thinking was that, given the lack of insurgent access to medical care, any wound to the trunk was serious and might well be fatal.
I don't know what shot sizes were used in this load but the idea was that the smaller shot would fill what would have been empty spaces between the larger shot and increase the probability of a hit.
But for HD? No thanks. I'll stick with 00 backed by slugs.
lpl
ETA: Here's one description, I can't vouch for its accuracy:
http://www.oldwardogs.us/martin_andrew/index.html
Saturday, 21 October 2006
Combat Shotguns in Malaysia
Contributed by Martin Andrew
During the Malayan Emergency the chance of a kill by weapon in very thick jungle, was during an ambush: Bren light machine gun, shotgun, M1/M2 carbine,no. 5 SMLE rifle, then the Sten/Owen sub-machine gun . Conversely during a chance encounter on a patrol it was the M-1/M-2 carbine, No. 5 rifle, shotgun, Bren LMG and the Sten/Owen SMG
During trials, and on operations shotguns produced more hits per exposure than any other weapons in patrol and surprise close quarter actions. The wounding effect of multiple hits by SG buckshot was very effective. The No.5 SMLE rifle was bolt actioned but the effect of a .303-inch, or alternatively a 7.62 mm round from an SLR/M14/G-3, was such that the terrorist was knocked down and finished off with another shot. The Bren LMG, or any machine gun for that matter, was the best weapon when preparing an ambush, providing a large amount of firepower and giving fire superiority.
In the Malaysian Confrontation the Royal Air Force flew Whirlwind helicopters out of Labuan on missions along the border with Kalimantan, Indonesia. The side door is on the right hand side, which is also the pilot's side. This meant that the left hand side of the helicopter was a dead zone as suppressive fire went. To remedy this a person usually interested in shooting, it was not a dedicated gunner, flew in the left hand cockpit seat and shot out through the window. The usual weapon was a Remington 870 but the preferred weapon was the Browning A5 borrowed from the New Zealand Special Air Service, who had removed the sear. This gave a full auto shotgun that was fired sideways so that the dispersion went sideways, due to recoil the weapon shot upwards on full auto. Shotguns were also fired sideways so that the empty cases did not end up on the floor jamming the rudder pedals.
The rounds themselves were zinc cased tropical ammunition and were from Fabrique Nationale (FN). The use of zinc-covered rounds was necessitated by the paper cartridge cases, common at the time, swelling up ane then being unable to feed causing a jam. Paper cartridges causing jams due to swelling had been a problem since the First World War, and were only cured by using metal cases. FN produced at least three types of 12gauge zinc shotgun cased rounds. Two had a case length of 59mm (2.3 inches) for use in shotguns with a 2.5inch chamber but the other was only 49mm (1.9 inches) long. All have the head stamp 12-FN-12. They are all have a brass stub with a cardboard case that is covered by a zinc sleeve.
The first one has a khaki cardboard case with a yellow wad. It has four SG lead buckshot over 28 B lead shot – the combination load developed for use in the Malayan Emergency. The other 59mm round has a blue cardboard case containing nine 00 lead buckshot. The 49mm round has the zinc rolled crimped over the cardboard case with a big yellow wad and contains number 4 shot. The 49mm case round is designed for 12 gauge shotguns with a 2-inch chamber. Twelve gauge shotguns with a two-inch chamber were designed for people who couldn’t handle the recoil of full 12 gauge shotgun loads. Two English gun makers that made double-barrelled shotguns for this cartridge were Purdy and Cogswell and Harrison.
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