First of all, I think the feed cover confirms it as an AGS-17, but with a new barrel or shroud. Secondly, it's undoubtedly fitted to an MG151 (GA1) mount in an Alouette III helicopter (Zimbabwe?).
Links:
http://www.saairforce.co.za/the-airforce/weapons/42/ga-1-20mm-cannon
http://www.imfdb.org/images/1/18/Ags17.jpg
http://img99.imageshack.us/img99/359/ga15px.jpg