Just verifying, since i'm told its supposed to apply to both - are both fletchette cannons supposed to be 500kg, or just the "MkI" variant? Right now, only the MkI has been reduced (at least, according to the information provided in-game).

That's odd. Sounds like a bug in the object management system.

This has been fixed for the Flechette MkII, but will need to be addressed in the future as an on-going bug with editing objects that have properties derived from a "parent". That's an internal-tool thing, but we've made note of the issue, thanks.