Hey good to see you're about summitfever
With 150mm long 5mm bits, if I start my hole with another short bit, then swap to the longer bit, I'm happy with the accuracy. One way to find out with 4mm.
Working with 2011 ally, it machines very nicely and I can go very slowly
I've a drill bit doctor that achieves very accurate drill bit sharpening/resharpening, should be ok shortening the bit, I managed to shorten a cobalt bits with it and kept the symmetry.
If one solid piece cannot be machined, I will be looking at two/three part assemblies.
The part I'm trying to recreate is four post up next to the ruler.