The reason was the date as indicated above. I own 4 or 5 and several are from Australia, France etc, and were made in the post Civil War era when the .22 LR was not even around in some cases. The BP was the issue. Keep in mind too that there were many cartridges used, many of which were quite a bit too powerful to have been needed for small birds. I have seen mention of these being used for rabbits, hare and small game; sometimes even small deer on the upper end of the power spectrum. The whole class seems to be a broad subject, but actually extends past England and small birds.
In later days, they were made in .22 and many of the medium bores were relined to .22. If you think about it, any of the Martini small bores are rook rifles by function.
