I've got a Spike's Nickel Boron BCG and it just won't close in the upper.  It won't fit over the gas tube.  The gas tube is dead center.  The gas tube length is correct.  I can "manually toggle" the BCG with a mallet so that it closes over the gas tube, but then it actually requires using a brass punch to drive it back out.  Even doing this a few times doesn't "break in" whatever is preventing it.  The Gas Tube is a spike's melonite gas tube.

I grabbed the BCG from my Colt and put that in the new upper....it closes just fine over that gas tube.  No issues at all.  Smooth as butter.  I also have a spare AIM Phosphate BCG and it also closes over the gas tube in the new upper just fine.  I took the Spikes' BCG and threw it in my Colt Upper to see if that would close...and while it does close....its "sticky" and the gas tube in my Colt is really worn in anyway.  There isn't much of a "lip" left at the end of it.

I looked in the Spike's gas key, and it looks fine.

Any idea what it might be?  Maybe some of the Nickel Boron coating worked its way into the Gas Key?  Maybe the Gas Key isn't quite straight?

Any opinion on whether maybe I just got a "bad" one and if I exchange it for another, you think I'd have better luck with a new one?  Or maybe the Spikes' Nickel Boron BCG just isn't going to work for me?  Seems silly that a Spike's BCG wouldn't fit over a Spikes' Melonite Gas Tube.

something would have to be out of spec, gas tube or gas key. if the colt fit fine one would have
to assume gas key is out of spec. could the nickle plating be to thick?

From your description, your spike's gas key is on the low end of the spec, and possibly even too small to be truly in-spec, combined with a gas tube that is at the high end of its spec.  Since the colt carrier has no problem closing on the spike's tube, the spike's carrier key is definitely the out-of-spec part.

It's possible that the fit issue is just some imperfections inside the spike's carrier key, which would normally break in over time, but that'll take a lot longer with melonited parts.

If you got both of these new from Spike's, I'd send Spike's the tube and carrier both, and ask them to provide you with an in-spec (or at least matching) pair under warranty for new parts. Or, you might just request that Spike's send you a new, in-spec carrier key if you feel competent to mount and stake it yourself.

ETA I see only the gas tube is melonited, so the key may well wear in to the gas tube in an acceptably short amount of time if you want to try that rather than sending in for replacement.

Measure the OD of the gas tube.
Measure the ID of the gas key.
Contact Spikes.

Over time I've seen many posts about NIB BCG's having problems.

NIB adds thickness to parts which sometimes causes problems.

Based on your post, I'd try shooting it to see if it will wear in, assuming that it will chamber a round if the CH is pulled to the rear and released.

Check and see if the gas key is bent downward just a tad. I see them fairly often where trainees drop them on the gas key and itll bend it down just enough to bind. The inside or outside of the key will not be bent or out of round, but the whole key will be bent down.