What is the purpose of the Ejector Screw?

[Neuraleanus]

It only comes into play if you have an FTF.
As hayboy says - the "drag" of the case rim on the inside of the body is insufficient to flick the weight of an unfired cartridge out. The ejector screw is specifically located so that the case rim reaches the tip of the screw just as the bullet nose on a MkVII round (with an overall length of 3") clears the chamber
If fitting a PH sight using the ejector screw hole ensure that you buy the correct replacement screw that is long enough to go thru the PH sight mount, thru the body sidewall and just protrude enough to do its job.

Yes, I've ordered a selection of BA threaded screws, which are not easy to find by the way, so I hopefully will be able to come up with something suitable.

Information

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[Alan de Enfield]

Yes, I've ordered a selection of BA threaded screws, which are not easy to find by the way, so I hopefully will be able to come up with something suitable.

The size you need is 3BA, but the length will be determined by the thickness of the sight mounting bracket.
The screw tip should have a slight angle ground onto it so that the 'angled flat points forward' when it is fully tightened up.
Be careful not to go 'too long' as you can get interference with the bolt and jam everything up.