I've been interested in armaments available to modern aircraft and I can't figure whether anyone still builds the old H.S.404 or an equivalent old fashioned 20mm cannon. Failing that, is a 20mm chaingun produced for the military market?
I am not aware of a 20mm chain gun. I've seen it in 25mm (shipborne and AFVs) and 30mm (Apache). I think that some exist in rifle caliber for coax mounts too.
Well, to be complete technically honest the M61A1 fits the category since is not a revolver gun but a rotating gatling gun.
But I guess you mean a reciprocating cannon.
2 examples come to my mind. The french M621 and the Rehimetal RH 202 , the first use the 20x102mm case and the german the 20x139mm, both fire at 1000 rpm.
RH 202
As curiosity the Argentine air force still use the HS 804 ( 20x110mm at 800rpm) and the Colt MK 12 ( 20x110 U.S.N at 1100 rpm)