LO,
Thanks as ever for coming back.
The change of gun on the F-35 was nothing to do with weight - my view from within the programme at the time (informed by more knowledgable BAE types) was not that LM thought they had 'weight to burn' - it was that they had no mechanism for controlling overall aircraft weight (e.g. a Chief Designer).
In any case, the gun change was a straight 'Buy US, not nasty European stuff' effort by GD, aided and abetted by compliant engineers within LM who knew better than to upset the USAF, who just love their Gatlings.
To be fair, there was technical risk with getting the Mauser's linkless feed system designed and tested, but the four barrel 25mm was a new design as well. There were rounds available at the time that met the requirement without DP, the best overall (in my view) was the Raufoss MultiPurpose (MP) design. Others can differ.
LO also makes a quite excellent point about integrated fire/flight control (IFFC). Tested by the US, fielded by the Russians and (I believe) the Swedes. Takes gun system effectiveness to a new level, and quite disregarded by the RAF. The GsH-301 is a good contender for best aircraft cannon around just now, in my view. Compact, light, reliable, and rate of fire good enough. The Russians understand this gun stuff quite well.
Best regards to all those cleaning the barrels,
Engines
PS (Edit): My understanding is that all cannon have been metric from day one - I believe because their design started in Europe (Hispano, Oerlikon). Almost all military gun calibres switched to metric by the 1950s, but some still use the older imperial 'legacy' calibres. This calibre history stuff can get quite geeky, so I'll stop now before I make a mistake....