I had a similar experience with the 50 cal. barrel I bought through the Barrel Exchange Program. Very fast (2 week) service, but the barrel had plastic Firesights which were hitting 2" left at 50 yards, despite the factory-set adjustments, and which moved with finger pressure. I couldn't put together a load combination which would stay in the black.
On my particular barrel, the rear plastic sight base seems to be pointing to the left, and most likely is the source of the problem. It was mounted with a standard slotted U.S. screw forward and a thin slotted screw in the rear. Not something I'd expect to see on a barrel fresh from the factory. But maybe it's not so fresh.
An order to Brownell's should be here within a day or so with a set of European (thin slot)screwdrivers and a steel Remington 700 sight base, which seems to be the match for the plastic one. I'm hoping it's just the base that's a ratty one from the bottom of the parts bin, warped off-center, and not an off-center screw hole.
It's not too expensive a fix, but I'd expect better from Remington for a $145 gun barrel. My guess is that they're cleaning house, using up old parts on an end of production barrel. We might just be buying the last remnants of the Huntsman line. Too bad. It's a nice, inexpensive, reliable little gun with a lot of potential, and it's finally getting an opportunity to be put to use this year in Massachusetts for the blackpowder deer season. This could have been a nice little moneymaker of a gun if more attention was paid to the quality of the sights and the breech plug design, another easy fix addressed much earlier in this forum.
Once the sight base issue clears up, I'll be giving the Williams Firesights another try. The metal ones this time, so the price of the fix will be climbing. I'll let you know how it turns out.