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adapterSimply put, a sub-gauge adapter (also known as a caliber conversion sleeve, cartridge conversion sleeve, supplemental chamber, chamber insert, subgauge insert, subgauge tube, gauge reducer, etc.) allows a shotgun to shoot a smaller gauge shotshell than its chamber was intended to shoot. Fitting inside the chamber of a shotgun, its external features mimic the shape of the shotshell the gun was designed to fire, while the internal features mimic the chamber of the smaller gauge being adapted to.

While sub-gauge adapters have been around for many decades, the first popular commercial product was the Savage Arms "Four-Tenner" which was an intermediate length (12") adapter for 12,16 or 20 Ga. Shotguns to shoot .410 shells. Since then, Little Skeeters™, GaugeMate™, Briley™, and Seminole™ and have all brought gauge adapters of various sorts to the market. In general, they work and pattern well. However, there is a bewildering array of styles and designs of adapters, some working better than others, which I will attempt to cover here.

Various Designs

There are custom-fitted full length barrel sleeves, mid length tubes and chamber-length tubes. They vary in cost from less than a hundred dollars a pair, to thousands of dollars for a custom fitted set of barrel-length sleeves. There are adapters that have elaborate and balky extractors, and adapters with no extraction system. There are adapters made from carbon steel, aluminum, stainless steel, and even titanium.


12-20-cyl-stsOne myth regarding chamber-length tubes is that they don’t pattern well. The truth is quite to the contrary, as can be seen in the patterns generated by chamber length adapters, such as made by GaugeMate. Compare a pattern from a full-length tube to a chamber-length adapter and you will find a remarkable similarity. The truth is, you don’t need expensive, heavy and semi-permanent full-length tubes to get great patterns from sub-gauge adapters!

In terms of performance, we have seen the tight patterns from chamber-length adapters like GaugeMate, and the obvious weight and balance benefits they provide, but there is still more performance reasons to use chamber-length adapters…

Shot Column Shape

Surprisingly, shot column shape is improved with GaugeMate adapters! Think of a conventional shot column from a 12Ga. Shotgun streaking towards a clay target, in the shape of an elongated cylinder. When shooting a 20 gauge shell from the same 12Ga. Gun (outfitted with a 12-20 Ga. Adapter) the shot column is less elongated and larger in diameter, more pancake-shaped! Thus, the pellets are positioned more efficiently to hit your target, there are fewer ”wasted”, redundant pellets covering the same footprint compared to the original 12Ga. Shot!

Shot Velocity

Yet another shocker regarding Sub-Gauge Adapters is shot Velocity. GaugeMate clocked speed improvements of more than 100 FPS with its gauge adapters. While some people will go to their grave denying this, some people also believed the earth was flat. The chronograph doesn’t lie. We even had independent labs verify it. The velocity increases come from less friction on the wad as it travels down the barrel. Think of the wad as expanding to the barrel size of the gun after it leaves the adapter. The wad in essence overextends, thought the wad is still stable and still seals with the barrel. The reduction in drag is significant enough to attain speed increases of over 100 FPS in many sizes of GaugeMate adapters tested.

Extraction of shells (pencil not included!)

As mentioned previously, there are vast differences in the way shells are extracted from the various styles of adapters on the market. Some adapters require manually removing the spent shells from the adapter and must be removed from the gun to reload. The aluminum Little-Skeeters™ product, for instance, requires a pencil or other poking device to push the .410 spent shell out of the adapter each time it is fired!

Ammo Versatility and Short Chambers

We have many appreciative customers who can use their old vintage guns again because they can now fit readily available modern 2 ¾” shotshells in them! Old shotguns, often European, commonly have 2 1/2” chambers. GaugeMate adapters have tapered ends to accommodate these shortened chambers, and allow 2 ¾” shell to be used in guns with short chambers!!! My old 16Ga. Parker, for instance has short chambers and I shoot 2 ¾” 20Ga. Shells in it. Not to mention that 20Ga. Is easier to find and a whole lot cheaper! Ammo versatility for your favorite vintage shotgun is a must.

Custom-Sized Gauge Adapters

customGaugeMate now offers a unique service to vintage gun owners, by offering made-to-order GaugeMate Custom sizes for obscure and odd adapter sizes. These are sizes that are too rare to inventory, so GaugeMate designs and fabricates them on a made-to-order basis. We can make custom adapters from 8 Ga. To .410 and all sizes in-between!

GaugeMate adapters can save hundreds of dollars, carry in your pocket, installs in moments, all while maintaining the delicate weight and balance of your fine shotgun. (full-length tubes add significant weight!) But most importantly, you can hit your targets better and more reliably than with other products. The added ammo versatility they provide is simply icing on the cake.

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Adapter Info

See GaugeMate's sub-gauge adapters in action in our Gaugemate Challenge Video
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Learn more: "How Sub-Gauge Adapters Work

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