Sentence:  The American firearms and ammunition manufacturers through diligent research and technical development have replaced the muzzle loader and slow firing single shot arms with modern fast firing autoloaders , extremely accurate <WSD>bolt</WSD> , lever , and slide action firearms . 
Meaning of the word marked with <WSD> tags:  a sliding bar in a breech-loading firearm that ejects an empty cartridge and replaces it and closes the breech