A few years ago while testing the patterns achieved thru back bored barrels, the thought occurred as to how much a barrel could be back bored and still maintain its velocity and patterning characteristics. We back bored a 12 gauge gun as large as it could safely be fired, which was approximately the same size as a 10 gauge gun. There seemed to be no loss of velocity and the gun patterned just fine. In the meantime, a lot of the new 3 ½ inch chambered 12 gauge guns came on the market with basically a 10 gauge barrel on them, so we knew that it was possible to push the limit farther, but it could not be safely achieved by back boring as there was just not enough metal left in the barrel to bore it any bigger.

12 Gauge Gun - 28 ga insert
20 yds - 3/4 oz. #9 Shot - Skeet Choke
1224 fps - 74% pellet count 30 in. circle

The only answer was to go in the other direction, which was to find a way to fire a smaller gauge shell thru a larger gauge gun in order to see what happened.  The best idea we came up with to accomplish this was to make a chamber insert tube that would allow us to insert a smaller gauge shell into the insert tube and them load the insert tube like a shell into the chamber of a larger gauge gun.

Our first test was to build an insert tube to shoot a 28 gauge shell thru a 12 gauge gun. This combination was as about as over bored as we thought could ever work. When the first shots were fired at the patterning board, we could not believe what we saw. The patterns were every bit as good as you would expect to achieve out of a 28 gauge gun. We assumed that the patterns were that good because there had to be a major loss in velocity making them very slow in order to hold together that good thru such a big barrel.
Chronograph tests proved otherwise. The shells were maintaining and sometimes even bettering the velocities we got from the same shells when shot thru a real 28 gauge gun. There was just no explanation as to what was happening. There was no way the 28 gauge wad could be sealing in the 12 gauge barrel and gases had to be blowing by. Based on everything we have been led to believe about shotgun ballistics, what was happening was supposed to be impossible. It worked, so we didn’t care what was supposed to happen.

12 Gauge Gun - 20 ga insert
20 yds - 7/8 oz. #8 Shot - Skeet Choke
1150 fps - 91% pellet count 30 in. circle

12 Gauge Gun - .410 ga insert
30 yds - 1/2 oz. #9 Shot - Full Choke
1244 fps - 65% pellet count 30 in. circle

The biggest surprise was that the patterns from the smaller gauge shells conformed to the 12 gauge choke constrictions. If we put in a 12 gauge skeet choke and patterned at 20 yards, we got essentially the same or better percentage of pellets within the 30 inch circle as we did when firing the same shell thru a skeet choke in a real 28 gauge gun. As the distance increased, the tighter chokes performed the same.

We knew since the 28 gauge inserts worked, it was a given that 20 gauge inserts had to work as well, so the next test was to make an insert for shooting a .410 shell thru a 12 gauge gun. This would be the real test, and to our astonishment, we got the same results. The patterns and velocities were the same as using a real .410 gun. The 20 gauge insert was next, and we were not surprised when it worked just as well.

The patterns were good and the velocity was just what it was supposed to be, so the next step was to see how it performed under actual field conditions. Off to the clays range we went to see if we could break any targets. After what we already knew about the patterns and velocity achieved in our tests, we were not nearly as surprised as the puller and our other squad mates were when the targets broke with authority. Even after seeing what was happening, their comments were that it can’t work because there has to be blow by occurring around the wad that should lose velocity and blow apart the patterns.

Although the chamber inserts broke targets, they needed testing on real birds. Tests in the dove field showed they could wipe those gray devils out of the sky just as good as a real gun of the same gauge. The same results were achieved on quail over pointing dogs.

12 Gauge Gun - 28 ga insert
30 yds - 3/4 oz. #9 Shot - LM Choke
1225 fps - 69% pellet count 30 in. circle

A funny thing had happened thru all of our testing. We had been trying to find out how big of a back bored barrel could be used before velocity and pattern percentages were lost, and what we discovered instead was a way to shoot smaller gauge shells thru a larger gauge gun. We could literally turn one gun into four different gauges of guns by merely inserting our chamber inserts into the chamber of the gun while utilizing the same chokes.

We had been led to believe by every thing we have been told about shotgun ballistics that in order to successfully shoot a smaller gauge shell thru a larger gauge gun that you have to have full length barrel tubes because it is impossible to maintain velocity and pattern percentages otherwise. All of our tests have proven to the contrary. Unlike full length barrel tubes that change the feel and dynamics of your gun by adding upwards of two pounds of additional weight to the barrels, Chamber MatesÔ weigh in at just a couple of ounces and provides your gun with the same feel and undisturbed balance, and with no noticeable increase in weight than when utilized in conjunction with the larger gauge shell that you normally use.

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