When the sabot slug hit the U.S. market in the late 1960s, it provided impetus for shotgun makers to design guns able to maximize the potential of these new rounds that could provide rifle-like accuracy out to a football field or more, effectively doubling the reach of the standard scattergun. This led to the Model 512.
Marlin's flirtation with slug-guns
Connecticut-based Marlin had been in the shotgun biz going all the way back to the 1890s. As the industry evolved so did the company, moving from black powder shotguns to smokeless guns with stronger steel. Then 16 gauge came and went as the 12 gauge rose to lead the market.
By the 1930s, the "European slugs" of German ammo designer Wilhelm Brenneke were being used in North America by knowledgeable gamesmen alongside domestic Foster slugs and individually made 1-ounce "pumpkin balls" of cast lead stuffed into repurposed hulls. These generally would stretch out the range of a shotgun from the typical 20-30 yards with the 2.5 and 2.75-inch buckshot loads of the day to 50 yards or so.
Then, sabot slugs (French for "shoe") which used a plastic sleeve to hold a bullet-like slug down the barrel and discarded shortly upon leaving the muzzle, came to the U.S. by 1966 and, with the promise of being able to make the vaunted 100-yard "football field" shot from a shotgun, makers began to make dedicated slug guns.
First up from Marlin was an incantation of their Model 55 bolt-action shotgun. The Model 55 was first introduced in 1954 by Marlin, and was a novel bolt-action design for the company.
(The Marlin 55S slug gun from the 1970s. You can see the 512's family resemblance)
Using a one-piece uncheckered American walnut stock with a pistol grip and butt pad, it looked more like a rifle than a shotgun. It used a two-round detachable box magazine that would hold standard sized 2 3/4-inch shells, and it seemed a good idea to make a subset of these bolt-guns to shoot slugs. Dubbed the Model 55S slug gun, it was equipped with 24" cylinder bore barrel with rifle sights replacing the more traditional gold-bead shotgun sight and just over 4,000 were made before 1983.
In the 1970s, the Model 120, long regarded as Marlin's best attempt at a modern pump action magnum chambered 12 gauge, was marketed complete with a 20-inch slug barrel variant. This barrel, like the 55S, had rifle-like sights and remained off and on in production until the mid-1980s. While it was nice, it could be better.
Enter the Model 512 Slugmaster
Designed in 1993, this gun took Marlin's past flirtations and pushed them to the next level. Taking the Model 55S concept, of a bolt-action shot 12-gauge with a thumb safety, they gave it a 21-inch rifled barrel with a slow and lazy 1:28 inch twist rate (the average rifle is more like 1:7, but hey, they aren't expected to impart a spin on a 1-ounce slug are they?).
Topping this off, the barrel had a rifle-like ramp front sight complete with hood and an adjustable semi-buckhorn rear sight-- something that you just don't see on your average shotgun. Further, if optics are your bag, it came drilled and tapped for a standard mount base (which was included) and both the front side hood could be removed and the rear sight folded down to produce a clean sight picture for a scope.
Also unlike your typical shotgun, the Slugmaster's Maine birch one-piece stock was pressed and checkered in the same fashion as Marlin's 880-series .22WMR rifles. In the end, the result was a bolt-action slug gun that was fully capable of that football field shot with the proper slug combination. We've personally seen about a three-inch three-shot group from the bench at 100 yards with Hornady SSTs and a matched optic.
The gun proved marginally popular for a time, particularly in jurisdictions like New York and Southern Michigan where hunters can use shotguns only (i.e. no rifles) for taking deer. It's been noted that the 1:28 twist is the same as used by many high-end slug guns such as the TC slug barrel
Getting your own
(In the last stages of the Slugmaster's life, it was polymer stocked)
Marlin intro'd the Slugmaster in 1994 and by 1998, it had morphed to the 512DL, which substituted a black polymer stock for the birch to help keep prices down. Then in 1999, this in turn became the short-lived 512P, which added fiber optic sights. However, by 2001, the more that century long run of Marlin shotguns was ended. Now, sadly, Marlin no longer makes a pump gun and with the company currently owned by Remington who markets the classic 870, odds are they may never again.
Nevertheless, that doesn't mean you can't get one of these Slugmasters that's been released out into the wild. They pop up in online classifieds and gun shows for around $300, which ironically enough was about their MSRP when new, so they at least have held their price point if you don't take inflation into account.
In the end, it's an interesting and very hard-hitting Marlin.