FWIW,
I just got a 25-06 back from Gardiner I sent them for a loose lockup with alignment issues--the rifle shot loose after less than 200 rounds of factory ammo. It came back locking up nice and tight and shooting nicely.
One thing about NEF's customer service--it's the best I've dealt with.
I have an idea that there may have been a batch of soft, for whatever reason, underlugs that were put on guns and shipped out. When I asked what had been done to my rifle, before I'd gotten it back, the woman at CS said it had been "muzzled and targeted". They most certainly must have done more than that; I think the underlug was removed and another one welded on to correct all three issues--the alignment, the lockup, and the fact it had shot loose. I have no facts to substiantiate this idea but the theory definately fits my observations...
In the unlikely event that your newly-fitted barrels do not lock up properly, you can be sure that they will "make it right" for you.