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GUIDE TO SHOTGUN BARRELS

My experience selling shotguns suggests that a prospective client’s eyes are usually drawn towards two features: the wood and the engraving. Gunmakers have long known that attention can be swayed easily by pretty, contrasting details and a slab of exhibition-grade walnut attached to the end of a gun is always a crowd-pleaser.

Similarly, the subtle art of engraving and the skill that the best craftsmen can demonstrate on the metalwork of guns can be truly astounding. Beyond the aesthetics, however, engraving and pretty wood don’t necessarily make a shotgun any better. Your pattern won’t be affected by upgraded wood, nor will your swing be impacted by the intricate engraving on the lockplates. It’s strange then that so many clients pay so much attention to this and so little to the business end of their prospective purchase – the shotgun barrels.

Perhaps I’m being overly harsh but, more often than not, interest in barrels extends only to a few questions: length, choke and, sometimes, chamber length. Although some technical aspects of barrels can be less than riveting to discuss – bore dimensions and forcing-cone lengths, important though they are, spring to mind – barrel-construction knowledge is too critical to be overlooked.

Making a set of shotgun barrels can be a long and incredibly time-consuming process that would require acres of ink to describe thoroughly, but gaining a basic understanding of it can be useful to the new buyer because it will help them evaluate the quality of the gun that they’re looking at – and what the salesman might be saying.

BORING AND DIMENSIONS

Standard shotgun barrels are smooth-bored and designed to fire multiple projectiles at once (shot) or, less frequently in the UK, a single projectile. Internally, barrels are not simply a straight tube but essentially have multiple constrictions within them, tapering down to the muzzle. The chamber houses the cartridge and contains the initial force when it’s fired; it will have the correct internal dimension to hold the desired cartridge calibre snugly. Forward of the chamber will be the forcing cone. This constricts the diameter of the chamber down to the end of the cone until it is the same diameter as the bore. The exact internal bore dimension will differ, depending on the maker’s or end-user’s preference, but there are set parameters and limitations for the internal bore to measure correctly so the gun is within proof of being a 12-, 16- or 20-bore. The final constriction point is at the muzzle and anything greater than the internal bore measurement will be classed as a choke. Each barrel tube will also be converging inwards, so that the shot column from each should come together at a set distance, just like a double rifle.

Key features on the outside of a set of breech-loading barrels are a hook, or hooks, to let them rotate on a pin or hinge disks, and the all-important lumps to engage with the bolt and hold the barrels in place securely. On a set of side-by-side barrels, hooks and lumps are more obvious structures, with over-and-under barrels generally having them incorporated on either the floor or sides of the breech end. These are necessary key parts to breech-loading barrels and the construction reflects that.

The simplest and easiest way to construct a set of barrels is to attach the hook(s) and lumps onto two separate tubes. Traditionally, this technique – mostly used on side-by-side barrels – would be called ‘round-bar barrels’. The barrel-maker would take two full-length barrel tubes and they would be brazed onto a separate piece of steel, which would be filed or machined into the hook and lump assembly. There are different ways of adding the lump assembly onto the tubes and different manufactures may name them differently as a result. ‘Platform-lump’ barrels generally refer to the lumps being brazed onto the underside of the breech with a curved contour to mate the curved underside of the rounded barrels. ‘Dovetail-lump’ barrels will have a dovetailed slot machined into the underside or floor of the two halves of the barrels, with the lumps slotted into place and brazed. ‘Through-lump’ barrels would have an entire piece of steel brazed between both tubes at the breech end, giving the greatest surface area of material in contact with each tube.

CHEAPER OPTION

Historically, poorer-quality guns were made with ‘round-bar barrels’, because they were easier and cheaper to manufacture. Adding anything structurally has a drawback: the weakness where the lumps are attached to the tubes. This can, in theory and under extreme circumstances, result in the lumps coming away from the tubes. However, while this may have been an issue in the past, it’s an unlikely scenario with today’s manufacturing techniques. Thanks to modern methods of joining metal, there are no real negatives, so you can have a seamless set of barrels without visible joins and still maintain strong tubes with an unhindered molecular structure of the metal from breech to muzzle.

That said, there is a simpler way of incorporating lumps into the barrels: by making them part of the original tube forgings. If each barrel tube can be forged to leave material to form half a lump and still be integral at the breech, then both of the two halves of the lumps can be joined together to create a set of barrels. This is the most costly and time-consuming method but is still the one used by best-quality manufacturers to make ‘chopper-lump’ barrels, so named after the distinctive appearance of the original forgings, which vaguely resemble ‘choppers’ or axes.

These barrels only have one join and are technically stronger than round-bar barrels. As each tube carries half of the finished hook and lump, both ‘halves’ of the barrel also take the stress and strain of loading and firing, and the lumps physically cannot separate from the tubes. Molecularly, the structure of the steel flows from the breech to the muzzle unimpeded and includes the lumps in that strength. Without needing to add additional material to form your lumps, the distance between the two chambers can be smaller, resulting in less space needed between the firings pins, less material in the action, as well as the barrels, and therefore a lighter, balanced gun. The tell-tale sign of the best chopper-lump barrels can be the incredibly fine line running between the lumps of a side-by-side barrel (trying to find the join between an over-and-under’s barrels can be slightly more difficult).

A very similar manufacturing method to chopper-lump barrels is a technique widely termed as ‘demibloc’. Essentially, the initial method is the same: both individual tube forgings carry the lumps and both halves of the barrels are joined together with a singular joint. However, instead of only brazing the two halves together, a male and female dovetail will be machined into the two halves, which slide together before being joined. This process makes an incredibly strong set of barrels, as the tubes have all the strength characteristics of chopper-lump barrels but are also joined together with a greater surface area and an integral bond. The downside to demibloc barrels affects side-by-sides, primarily, as the extra space needed to have a vertical dovetail machined in, although minimal, does necessitate more material. With the additional material, the distance between the chambers is greater, giving a larger action and more overall weight. This can be much less of an issue on a set of over-and-under barrels, as the dovetail can be horizontal between the two respective chambers, making the two tubes near inseparable when complete.

But the most common method of manufacturing modern shotgun barrels, and one that is widely used for the majority of new over-and-unders, is ‘monobloc’. As the name suggests, there is a singular block of steel, into which two individual barrel tubes are sleeved. Although ‘monobloc’ is a modern term, the key manufacturing process is old, dating back to famed Belgian gunmaker and inventor Henri Pieper’s patent for a ‘one piece steel breech’ in 1881. With modern techniques, monobloc barrels are the simplest and easiest way to manufacture a large quantity of barrels, yet make them to an incredibly high standard. The breech end, which the tubes are sleeved into, can be machined to incorporate the hook(s), lumps and extractor assembly, then finished with the tubes in place. The only real downside to ‘monobloc’ barrels can be the fine joining line where the tubes are sleeved into the block, normally forward of the chamber, although this can usually be concealed with a ring or border engraving.

Although all these techniques produce good-quality barrels, as a rule you will only see chopper-lump and demibloc barrels on best-quality guns. But don’t be disheartened if your next potential purchase incorporates a different barrel design. Just make sure that, armed with your newfound knowledge, you give those tubes a more than a cursory glance before admiring the wood and engraving.