receiver impingement

[DRUN]

I am currently working on an AR build. When I attach the upper receiver to the lower receiver, the part of the upper receiver for the retaining pin is rubbing against the back of the receiver. I have to slide the upper receiver form the front to the rear over the lower in order to fit the two together.There seems to be a lip near the aft of the receiver (see photo). It is a Dalphon lower receiver. As I understand they were manufactured by either Olympic Arms or MMS. The upper receiver is a Cerro Forge. A gunsmith recommending grinding down the lip. I just wanted to know if this is the road I should be taking or addressing the upper receiver. Thanks in advance.

[ar15barrels]

Take a close-up picture of the upper receiver's rear lug.
There's usually a generous chamfer on the rear corners of the lug to clear that radius in the lower.

WECSOG rule #1 is always modify the part that's easiest to replace.
In this case, that's the upper receiver.

[Seesm]

I am currently working on an AR build. When I attach the upper receiver to the lower receiver, the part of the upper receiver for the retaining pin is rubbing against the back of the receiver. I have to slide the upper receiver form the front to the rear over the lower in order to fit the two together.There seems to be a lip near the aft of the receiver (see photo). It is a Dalphon lower receiver. As I understand they were manufactured by either Olympic Arms or MMS. The upper receiver is a Cerro Forge. A gunsmith recommending grinding down the lip. I just wanted to know if this is the road I should be taking or addressing the upper receiver. Thanks in advance.

Drun from your picture and most AR's we have built it appears that lower rear pocket is not machined correctly... It is a very easy fix however with a manual knee mill and can then be re-anodized if you care to (not a bit deal since you will not see the newly machined area)

Pm with any questions.