Simply 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.
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.
One 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…
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!
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.
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!
Another “remove for reloading” adapter product is GaugeMate Silver, an all stainless-steel, economical product line. It was our inaugural product in 2002, to rave reviews. GaugeMate Silver adds Patent-Pending finger-tip cutouts on the face of the all the .410 adapters for ease of re-loading. No other competitor has that important feature.
The Savage “Four-Tenner” and other adapters contain a built-in extractor that slides back and forth with the extractor/ejector of the gun. In theory, this is a great idea. Unfortunately, sliding extractors have a serious reliability problem from the clearances of all of the components involved. There is the clearance of the shell, adapter, the adapters’ extractor, the gun’s extractor, and of the chamber! All of those clearances add up to frequent jamming and jumping of the extractor when the gun is closed, making a real mess of things.
Sliding extractors are limited to sizes of adapters that are thick enough to have sufficient strength to function properly. For example, it would be impossible to offer an extractor in a 10-12 adapter, as there is only .018” wall thickness. This drawback limits the variety of adapter sizes that are available with extractors.
GaugeMate offers a unique, patent pending solution to the whole idea of a “Stay in the gun” adapter and is very economical as well. Rather than a moving extractor built into the adapter, GaugeMate designs their all stainless-steel GaugeMate Gold adapters to slide with the extractor. In other words, the whole adapter moves with the gun’s extractor/ejector, so when the gun is opened, the adapter “presents” the spent as an extractor would. Durable O-Rings provide sufficient friction to retain the adapter in the chamber. A portion of the adapter’s rim is cut away to reveal the rim of the shell, for easy and efficient removal. Simplicity and reliability at its finest…
For thin-walled adapter sizes, GaugeMate Gold has another innovation that makes these sizes a reality. A thin-walled adapter, such as the 10-12, must act as its own barrel, as the clearance of the chamber to the adapter means that the chamber does not support the adapter strength-wise. Some competitor’s adapters actually bulge and become un-useable from the pressures of the fired shell. Simply by slotting the adapter along its length, the GaugeMate Gold adapter is allowed to expand to the inner wall of the chamber providing great durability, eliminating bulging that is found in some competitor’s products.
The GaugeMate Gold series uses the unique slotted feature to provide a convenient way to retain the “stay in the gun” thin-walled adapters in the chamber of the gun. We simply spring open the slotted adapter to create a friction fit in the chamber of the gun. The rim is partially cut away to expose the shell for easy removal. It’s such a simple and elegant solution. This is one of the many reasons why GaugeMate is able to offer both the GaugeMate Silver and GaugeMate Gold product lines in 14 different sizes, and why we have the number one selling gauge adapter on the market!
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.
GaugeMate 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.
See GaugeMate's sub-gauge adapters in action in our Gaugemate Challenge Video
Learn more: "How Sub-Gauge Adapters Work
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