Mcmillan's shank length measurement is measured from the action face to the end of the full diameter shank that you wish to have (by shank I mean barrel tenon - the thick part. Don't mind those talking about turning down the shank they think you mean something else I believe. Remember the measurement includes the recoil lug.
On my last 2 Mcmillans I ordered a 1" shank and specced a .250" recoil lug. So that means I'll have a .250" lug and then .750" of full diameter barrel shank.
The chamberings that I shoot are no where near as big as a 338 so I could see you extending that a hair to 1.25" or even 1.5" but no less than 1". 1" to 1.25" might be just right.
When you speak to the gunsmith give him the same request for your finished shank length, or simply say "cut shank length to fit stock inlet".
Trigger tech for a trigger.
