Puszkarz does mean 'gunsmith, gunner', but also 'maker of boxes'. I think this meaning may be the older. In German, Buechse means both 'box' and 'rifle'. In Dutch, bus has the same two meanings. So there does seem to exist a link between 'box' and 'gun'. Given the similarity between 'Puszk' (the -arz ending denotes an occupation) and Latin 'Pyx(-is, late Latin Buxis)' I think it is reasonable to conclude that this name derives from a Latin word for box.