Fitting & machining of bracket pads blanks on a No#4

[BrianLara400*]

Hi all, apologies for the delayed reply - have had a hell of a lot of other commitments lately, hopefully I can get some machine time soon.

I think I'd be most confident of the resulting spigot being within tolerance using the radial milling/ dividing head solution as per Peters suggestion, it'll certainly handle the wholesale material removal better than any specialty boring bar I can grind up (its effectively an interrupted cut and body of bar itself would need to be less than .115" - absolutely miniscule)

Using the dividing head is not without problems of its own though @ the set up stage. Its got plenty of angular adjustment however this can only be used to bring the spindle axis into parallelism, to use any of this inbuilt correction to plumb up the No4 action etc would cause it to turn the spigot conically (if only minutely) and be a PITA for the subsequent machining of the rear pad.
Put plainly this means what ever fixture I devise to hold the action for machining, will require (angular/planar) adjustment of its own independent of the faceplate & dividing head it sits on.

Also to allow the dividing head & action through 360* of rotation, The barrel would need to be pulled off so the assembly clears the column of the milling machine.

Does anyone have any specific objection to using an action mandrel to indicate off for the purposes of collimation?
This may be blasphemy/heracy etc, but having the barrelled action setup between centres and expecting every quasi important point between to fall on that imaginary centreline isn't ideal to my way of thinking.
The barrel would likely be replaced in the future anyway @ which point the receiver ring would be faced, threads trued and barrel blank set up to suit the bolt axis, so surely this would be the logical reference to set the bracket pads to and collimate the optical axis to?

flame away

Also those of you who've made & used the "hold down" plates for No#4 action work, do you find the act of judiciously "dogging" a receiver down causes noticeable distortion/deflection?