Amerigo Cei-Rigotti was a major in the Italian Bersaglieri (light infantry) in 1900, wen his innovative self-loading rifle design was first introduced. Unlike many or the very early semiauto rifle designs, the Cei-Rigotti is a light, handy, and pretty compact rifle:

The rifle was select-fire, and chambered for the 6.5x52 Carcano cartridge. It shared the size and style of the Carcano as well as a few small parts, but was built from the ground up and was not a conversion of a bolt action rifle. It operated via gas pressure on a short-stroke piston under the barrel. This example has a small magazine, but several different sizes were used in various tests, reportedly up to 50 rounds in capacity. The magazines are not detachable, though, and must be reloaded with stripped clips through the receiver.

Ultimately, the Cei-Rigotti was not adopted by any military force despite being tested by several countries over the decade after its introduction. Most folks today say this was due to erratic and unreliable functioning, but we have not seen any test reports from the period substantiating this (and the mechanism seems quite sound to us).