Gunsmithing Work holding for AR uppers?

[Mech_eng]

I’m not a gunsmith by any means but I am a home shop machinist. I have a little Grizzly 6x20(G0463)manual mill, a Clausing 10x24 lathe and a Tormach 770 CNC mill. I have a coworker(we are both essential ? so we are already cross contaminated...) that wants to open up an ejection port on an AR upper. Can anyone provide any help on how to hold the upper at the right angle while minimizing coatings damage? Thanks in advance!

 

[357Max]

I have a similar setup less the Tormach (jealous).

I would probably dig out an old set of 1" pic rings, mount them to the upper, and clamp a 1" solid bar of whatever I have laying around.
Now you can clamp the bar in the mill vise @ whatever angle you want to mill the port.

 

[357Max]

I don't like tilting the head on my mill cause it's a royal pain in the ass to get it dialed back in.

 

[Easy_E]

I opened up a bunch of AR uppers for 450 bushmaster builds with a vise like this . Put a piece of aluminum flat stock between the ears and tighten squeezing the flat stock and rail . To get the angle use the angle blocks from you mill hold down set .