Anton Politzer of Mauser, late WW2. Designed the first automatic revolver cannon, the MG 213C, copied as the Aden/DEFA and now adopted by the USA for the JSF (Mauser BK 27).
I notice that Kalashnikov is the only Russian to get a mention, but there were lots more. In fact (just to be controversial) when it comes to larger-calibre automatic weapons (especially aircraft guns) they have consistently produced the best designers of most of the last century. Just a few examples:
Berezin, Shpitalnyi, Nudelmann, Rikhter, Gryazev, Shipunov.
The last two have designed a phenomenal range of successful weapons of various different types; the GSh-23, GSh-30 and 2A38M twin-barrel guns, the GSh-6-23 and GSh-6-30 rotary guns, and the GSh-301 recoil-action used in the MiG-29 and Su-27. These are all astonishing performers in terms of rate of fire for the weight.
Tony Williams
Military gun and ammunition website: http://www.quarry.nildram.co.uk
Discussion forum at: http://forums.delphiforums.com/autogun/messages/