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A Comparison of Different Gauges of Shotguns
There have been many different choices of shot gun gauge over the years. The most common gauge that many people are familiar with is the 12 gauge. This has become the best selling shotgun gauge. In addition, some other popular sizes are 10 ga, 16 ga, 20 ga, 28 ga, and 410. Those might be the more common shot gun gauges, but they are not the only ones. On the bigger end of the spectrum, there have been guns made that were 8 gauge and even 4 gauge (which had over 1" diameter bore)! Some other shotgun gauges available in the past include 24 and 32 gauge. These obscure shotgun gauges are not readily available anymore. For a visual comparison of shot gun gauge, the picture below shows the most common gauges and how they compare to each another. Keep in mind that depending on your computer monitor and its resolution settings, what you see on the screen might not be actual size. Even so, you can still get a relative idea of how the different shotgun gauges compare to one another.
The bore sizes listed above are fairly standard in the industry, but you can find some variations. Over the years, some companies have offered over-bored shotgun barrels (sometimes also referred to as back-bored). It was believed that a slightly over-bored barrel could help improve overall performance. For one, it was said to reduce the felt recoil of the gun. Some of the gases from the burning powder charge could escape around the wad and this was said to reduce the pressure slightly and therefore reduce the recoil. This has been confirmed by many people. Contrary to what you might think, the over-bored shotgun barrel has also been said to slightly increase the velocity of the exiting shot. You would think that if the barrel is over-bored and some gases are allowed to escape past the wad, then the velocity would be less. However, the velocity might be slightly increased simply because of a slightly looser fit and the reduced friction between the wad and the inside of the barrel. Apparently, it has also been found that a over-bored shotgun barrel can also improve the shot pattern by resulting in less shot deformation as the wad travels down the slightly larger bore. As a result of these benefits, some shotgun manufacturers have chosen to make their barrels with a slight overbore. What this means is that you might find that actual shot gun gauge bore sizes can vary some from gun to gun.
To give you an idea of how the power of each shotgun gauge compares to each other, below you will find some ballistic data on shotgun slugs. The ballistic data shown below is for standard Federal Power-Shok slugs, with the exception of the 28 gauge load which is a Brenneke load. The 28 gauge is not as common, and so there are less companies that offer ammunition for this round. Brenneke is one of the few companies that actually offers a factory loaded slug for the 28ga. One of the things you might notice is that there is no real benefit in using a 10 gauge shotgun - at least when comparing these shotgun slugs. The 12 gauge standard length 2-3/4" Federal Power-Shok shell launches a lighter 1-1/4 oz slug, but it is travel at a much higher velocity. The result is the ballistic energy at short ranges is actually comparable to the 10 gauge slug. If you want even more power, Federal offers a 12ga 3" magnum load which propels a 1-1/4 oz slug at 1,600 ft/sec with a whopping ballistic energy of 3,109 ft-lbs. That's a lot of energy! By the way, for the sake of comparison the velocity and energy specs given are measured at the muzzle of the gun as the slug is exiting the barrel.
BALLISTIC ENERGY COMPARISON OF DIFFERENT GAUGE SHOTGUN SLUGS
In case you didn't know, the way that they came up with shotgun gauge designations was related to the weight of a lead ball that would fit inside the bore. There are a couple ways that you can look at it. Let's take 12 gauge for example, since it's the most popular shot gun gauge. A 12 gauge shotgun will fit a single lead ball in it's bore that weighs 1/12th of a pound. Another way to look at it is that it would take 12 lead balls with a diameter that would fit within a 12 gauge shotgun bore to equal 1 pound. So, that means that a 10 gauge shotgun bore would hold a lead ball that weighs 1/10th of a pound. A 20 gauge shotgun bore would hold a lead ball that weighs 1/20th of a pound... and so on. With shot gun gauge, the bigger the number, then the smaller the bore size. Hopefully that all makes sense. The shot gun gauge sizing convention is old and a bit odd if you are unfamiliar with it, but it makes sense if you understand how they came up with it. The 410 shotgun is a bit different in that the .410" is the actual size of the bore. For comparisons sake, a 410 shotgun would be close to 67.6 gauge. Hopefully this helps clarify some of the underlying meaning behind shotgun gauge, and how the different gauges of shotgun compare to one other.