Mismatched feedramps

[mutt]

Are you sure you don't actually have m4 feedramps but the extension and the receiver are simply misaligned a bit? I remember this being an issue with PSA uppers in the past. I personally wouldn't accept an upper that had a M4 receiver but a rifle extension. That is the worst of the mismatches. Since you're in Centennial I'd have Rob at Bowers Tactical look at it for you to verify what you have. He could probably dremel fix it properly as well.