Here's what I have:
3 uppers, all the same model and from the same manufacturer
3 lowers, all the same model and from the same manufacturer (different maker from the uppers)
2 of the uppers fit properly with all 3 lowers.
1 upper won't fit properly with any of the 3 lowers. I can slide the pivot pin in fine, but the takedown pin won't slide in all the way unless it is forced (hammered in) and I don't want to have to do that. If I reverse the process, I can slide the takedown pin in first without issues, but now the pivot pin won't slide in with finger pressure only. This happens with that particular upper and all 3 lowers.
I sent the problematic upper back to the manufacturer with an RMA. Got an email back saying that they tried the upper with several other manufacturers lowers (except for a lower from the same company as the ones I have) and they all fit fine so they sent the upper back to me (awaiting delivery on my end).
What least destructive means can I resort to in order to get this upper to fit with any of my lowers?