But you're already talking about two very fine guns. Just because you have to consider trade-offs at that level doesn't mean there's no qualitative scale at all, does it?
Yes and No. Sure, there are guns objectively worse than both the M16 and AK47 in any context. But beyond trivial examples of almost implausibly bad knockoffs, a universal qualitative scale remains impossible to define.
Perhaps it would seem less wild a concept if framed as:
you can't build a truly useful qualitative scale until you know the context.
You can trivially sort things into 'good' and 'crap' - it just won't get you very far.
