From a quick Google
6mmbr.com/20Caliber.html
angelfire.com/sd/6mmackley/twentycaliber.html
tromix.com/Tromix_Tackhammer.htm
Tactical 20
The Tactical 20 was invented by Todd Kindler long before the .204 Ruger hit the market. Based on the .223 Remington parent case, the Tactical 20 is simply the .223 necked down to 20 Caliber with a 30-degree shoulder. Some versions have slightly less body taper as well--it's wise to compare reamer prints. Although the Tactical 20 has about three grains less case capacity than the .204 Ruger, it delivers very similar performance. With the right load, you can get Max velocities with 32- and 40-grainers that give up little or nothing to the .204 Ruger. Varmints probably won't know the difference. The Tactical 20 is easy to load and exceptionally accurate. Currently, Cooper Arms of Montana and Dakota Arms offer factory rifles in Tactical 20 chamberings.
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