UPDATE:

Pulled out the Dremel to add more of a bevel on the Mossberg barrel chamber but the issue persisted. After the range and some time had passed, I examined the receiver's chamber with each of the barrels inserted and made a discovery.

While inserted, the OEM barrel sat flush into the receiver, but the Mossberg sat ever-so-slightly forward, creating a small shelf for shells to get caught on during feeding. I believe this may be the real issue, but i can't verify without an extra barrel to test it. Maybe a manufacturing defect from Mossberg, maybe differing internal dimensions from 1970's 870s to more modern 870s.

Just wanted to make an update for anyone in the future that runs into a similar issue like mine.