20BR is one of the many 20 caliber rifle cartridges known as wildcats (not commercially available). It is basically a .22 BR Remington case necked down to accept a .204" diameter bullet maintaining the same shoulder angle and overall length. It was primarily created to combine large powder capacity with a short efficient case design thereby giving superior performance to all other 20 wildcat cartridges available. Although there are some cartridges which came close;.20ppc, .20 Tactical etc. Combining the ability to propel a 39/40 grain bullet at above 4000fps and the inherent accuracy of the BR case design, the 20BR quickly became the preferred flavour of many varmint hunters and target shooters alike. Other advantages of this cartridge include; its ability to suite any rifle action which has the 30-06 sized bolt face(fitted with the appropriate barrel and chamber size of course), its short length which enables action makers to utilise shorter ports in their actions promoting rigidity, easily formed cases and good reloading characteristics.
Some drawbacks of this cartridge include; it is not commercially available and rifles must be custom made, some would argue that is an overbore (barrel burner) cartridge and therefore is not practical when compared to less aggressive 20 wildcat chamberings.
In more recent times some other wildcat cartridges have come about which exceed the ability if the 20BR without being excessively larger or impractical, notable amongst these are the 20 Fergusson Ace and the 20 Magna which both have their origins with the 20BR.