The 2.7mm Kolibri Auto as compared to the .22 Long Rifle
The 2.7mm Kolibri Auto is (as far as I know) the smallest commercially available centerfire round that has ever been produced. It is the idea of Franz Pfannl, an Austrian watchmaker and was patented in 1910 and introduced in 1914. It was fired from a smooth bore Kolibri pistol.
The round is 3 millimeters at its widest point, and 11 mm from the meplat to case head. The round headspaced on the case mouth. The bullet itself weighed 3 grains and averaged a muzzle velocity between 650 and 700 fps, creating about 3 foot pounds of energy.
There is some debate on another round called the 3mm Kolibri. It is very similar to the 2.7mm Kolibri but uses a lead bullet while the 2.7mm uses a jacketed. The 3mm is also physically larger than the 2.7mm but not by much and performance of the two are nearly identical.
The rounds and pistol were discontinued in 1938.
Kolibri, is the word for hummingbird in several languages and is where the round gets it’s name.
Given the 25 Auto develops about 73 foot pounds of energy and is less than ideal for a defensive round these Kolibri pair would be sufficient for game up to and including a rabid frog or wounded mouse. With that said, as with any firearm it can be very dangerous.
A rabid frog can be very dangerous, never under estimate your adversary.