You are spot on about shaft diameter and once you starting going smaller than 12.5mm, the weight of the shaft gets
lower unless you artificially added weight. That’s easily understandable since there would be less mass. I had 3 noteworthy cue makers advise me the KW shafts I was looking to have built couldn’t be made without adding an insert.
I scoffed at that notion and just kept searching for cue makers. I mean it is just common sense that all you need is a heavier blank which means being selective with your wood inventory. The fact some cue makers do, whereas a lot of cue makers apparently don’t, have a diverse wood inventory is likely a contributing factor, if not the underlying reason, why I was misinformed by many cue makers I spoke with. Well, I have the shafts in my case & shaft weight does matter. One of my flat faced KW shafts is 12.55mm, 4.1 ozs and is just tip, small ferrule, phenolic black 1/2” collar & KW wood.