Upper receiver lug not fitting into lower

[gesundheit]

As the thread title says the upper receiver is not fitting in the rear with the lower (see the attached picture of where the problem is happening). The upper receiver lug is a little wider than the lower cavity.

I have two questions that I need your help with:

1. What are the receiver lug dimensions per mil-spec so I can compare?

2. What part - the receiver lug or lower receiver needs to be filed to get the upper all the way down the receiver?

[ott1]

I would take some material off the sides of the lug instead of removing material from the pocket in the lower.

[Redchevyman]

The pocket should be .500" and the upper lug should be .495".
What brand lower and upper are you working with?

[gesundheit]

Thanks!! I will measure and report the result. The lower is generic one, the upper I am waiting for response from the vendor to give them the benefit of doubt. Don't want to drag their name in the mud if the lower is out of spec.