How tight should magazine cap be on a 1978 Wingmaster 12 gauge?

[John A.]

Tighten it until it stops turning with hand pressure. Don't worry about worn blueing or paint or whatever is on it.

This is what keeps the barrel from moving when the gun is fired and essentially helps lock it in the receiver.

I would expect wear on a 40 year old shotgun.

[Synchronizor]

Like John A. said, a firm hand-tight is fine. As long as you have the mag cap detent in place and moving properly (you mentioned "clicks", so you should be good), that's all that's needed for the cap to stay in place. One note I'll add is that if you tighten the cap while the barrel is hot, it's a good idea to re-check it once the barrel cools and tighten it down further if necessary. That doesn't come up too often, though.

Finish wear under the mag cap is normal, and nothing to worry about.