I just bought a Lilja-Nordic barrel and I am sticking it in a Vltor MUR.
Here is the result:
Notice the left ramp is slightly off, this creates a small, sharp burr on the outer edge that looks MUCH worse when viewed from the tail of the upper - it’s hard to get a good pic because of the angle.
I’d run it, and if there are issues I’d return it and ask them to fix the extension, if they won’t I’d either replace it myself or in a worst case scenario, attempt to de-burr it with a dremel.
Burrs in the barrel extension feed ramps are pretty common. They can also be so small that the human eye cannot see them. The bullet then drags on them causing a mafunction. This is why it normally a good idea to ALWAYS polish the ramps.
It it were mine and it either was a problem or I couldn’t live with it I would pull the barrel and use a small gunsmithing file to take the burr off so as not to hit the aluminum upper, reassemble, and check for fit, then shoot for function.