The Winchester Winder Musket was (despite its name) a training rifle, built by Winchester and used by the United States between 1904 and 1922. The Winder was designed to be as long and as heavy as a service rifle, but was intended to be as cost effective in design and usage as possible.
Despite its name suggesting that the Winder was a musket, in truth the Winder (which was named after it's designer Col. C.B. Winder) was further from a musket than most other firearms in use by the United States at the time. The barrel was rifled (albeit very slightly) and used a breech-loading mechanism pioneered by John Moses Browning: the falling block.
The shape of the falling block mechanism was the most distinctive feature of the Winder (other than the fact that it fired a significantly smaller cartridge than a service rifle, despite weighing the same), in that it came to define what type of Winder the shooter used. Early Winders were also refered to as "high walls" because the receiver area around the falling block featured a high edge. Later Winders were often refered to as "low walls" for a similar reason.
As eluded to above, the Winder fired a significantly smaller (and cheaper) cartridge than a service rifle of the same size. It's chosen calibre cartridge, the .22 Short , was cheap to produce and allowed trainees to learn how to cope with the weight and length of a full size rifle.
The Winder Musket had originally begun life as a passion of Colonel C.B. Winder, whom wanted to train marksmen using full size firearms which used smaller calibres (which, he hoped, would improve their accuracy as well as make training costs cheaper). He combined this concept with the most popular breech-loading method of the day, Browning's falling block, to create the Winder.
The Winder served as the United States' primary rifle training tool from 1918 until 1922, distributed between various areas of the US military and their respective training sections. Once the Winder was superceeded by a new design in 1922, the Winders were sold through the DCM to fund the purchase of the new training rifle, the Model 1922.
Henshaw, Thomas, "The History of Winchester Firearms 1866-1992" Hartman, Bert, "The Winder Muskets"