I went through this with a Winchester Model 70 just a few weeks ago and it was a tricky job. Instead of grinding an end mill down to the #8-40 pilot drill diameter, I cut off a #28 high-speed drill bit just where the flutes start and sharpened it flat. Chucked up close in the milling machine, it served as an acceptable cutter but could've been better. If I ever do this again, I'll grind a 5/32" end mill down to .140" and make the job easier. I've heard this 8-40 job referred to as the M40 scope base upgrade, as it's supposedly been done to every M40 sniper rifle the USMC has ever built. In spite of that, I don't know of a single 'smith out in the private sector who does it.
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