Blackie,
I agree, and am totally aware of the limited real world application of this issue.  Having to leave the gun in the car when going into a mall (signs will keep the bad guys out I'm sure) comes to mind.
But it made me curious.  Might increase knowledge of this fun little gun if I figured it out.
Truth be told, it only happened once using the freezer to see if it would repeat the issue.
Every time since, leaving it longer, keeping the magazines in with it as well, have not yielded the same result.
It's been suggested via PM that perhaps it was due to gummed up lube.  That's possible but I'm not much of an over-oiler.  It was clean before the range trip, and clean before the test in the freezer.
When it failed, it was only on NovX ARX.  Shot lots of different ammo that day OK.  Shoots Ruger ARX OK, but I didn't try them during that outing either.
I have nothing bad to say about the gun.
I'll post again on it if I learn anything new.  It's supposed to get very cold here next week.
Thanks,

I have it back in the freezer now.  Easy enough to simulate that way (don't need to be in single digits to try this). 
I have 5 magazines.  #1 and #2 are originals, #3 I purchased same day I picked up the gun in July, and #4 and #5 were shipped to me when I emailed customer service and whined about the nose-diving issue and they sent me the new part number that looks the same as the old one (the lips do measure differently).  1,2, and 3 all have shortened follower legs, and only #3 has had its funky front lip bent in and some metal trimmed away with a file.  I am using #1 and #2 only for this test. 
I'll pay closer attention to how the NovX rounds stay down in the magazine when cold and maybe try to describe them better.  May even put the magazines in the freezer with the gun on one of these tries.
I noticed but didn't mention that when frozen I had difficulty locking the slide back so I could look at the rounds inside.  Pushing up on the slide release didn't seem to catch the slide until my hand warmed up the gun.
I really like the NovX ARX design and concept and lightness.  Just happened across this issue by accident. 
I'll post results.

Only chance I had to get to the range lately was in the cold snap last week.  The R51 was running pretty well on different factory 115 Grain Round Nose types as well as a few Self Defense rounds (Gold Dot, Remington Ultimate Defense, etc.,).
Then I tried the new NovX ARX rounds.  First time, too.  Well, tried to try them.  Mind you, the temperature was about 19 degrees F and the gun was admittedly cold.  I put the magazine in the gun and slingshotted the slide.  Nothing.  The first round didn't even load.  Tried to slingshot again, and nothing.  The rounds weren't just nose-diving, they were submerged.  Removed a round from the magazine and tried again to see if it's just the spring pressure, and, still nothing.  Had to take 2 rounds out before the thing would load and fire. Fired OK.
Took it home and let the gun warm up inside.  Loaded the full magazine with NovX ARX in it and cycled through all the rounds no problem (gun pointed outside through a door in a safe direction, full rounds dropping safely onto a mat).
Figured I'd stick the empty gun in the freezer for an hour.  Put in a full magazine (magazine stayed at room temperature) of the NovX and same thing.  Rounds submerged.  Tred again, and this time, I think because I was inside and my hand was warm and the magazine was at room temperature, the gun warmed up and cycled through all the rounds no problem. 

Anyone test the temperature limits of this weapon yet?   Granted, in the cold the "normal" ammo worked fine.
Thanks,

Spoke too soon (there's a page 2):
https://www.gunpartscorp.com/products/1583190
Wow, $4.40 for an O-Ring. 

Found screws here:
https://www.gunpartscorp.com/products/1583110
Haven't seen O-Rings though...

Thanks for the link to the article! "Excruciating detail," yes, but the description of the "Pederson hesitation" lock finally gave me the clue I needed to understand why I had such difficulty reassembling the slide onto the frame.  I could see on mine that the slots inside the slide (the ones that mate to the frame front rails) are actually obstructed by the Pederson Breech Block when the it is held in place by the barrel/spring (as shown in the Remington video).  The Pederson breech block stops the slide right at the front rail due to its obstruction of the slots when I try to reassemble.  I had thought the rails needed to be clear in order for the slide to go back onto the frame, but that's not quite right.  You simply push and wiggle (the slide only—don't push on the barrel in any way) and the slide will move a tiny bit beyond the block while the block is held by the front edge of the frame rail.  This allows the slide to push the block down same way as it does after firing (as described in the detailed section of the article on the Pederson action).  The now dropped block clears the slots in the slide to allow it past the frame front rails.  Best to be holding down the ejector as you do this, or it may pivot upwards and you have to start over. 
Once I got a feel for it, it was easy to reassemble.

Same here.  They emailed on 8/21, saying that the magazines were back ordered.  Part number of the magazines in the parts order pdf they sent is the new number.  No word since. 
I may call in a while to see if I can get more info.

I'll let you know what I learn.

First difficulty I encountered in taking the R-51 down for cleaning was an amount of nuance in the manual.  The instructions tell you to simply "pull the slide rearward until the Slide Stop is opposite the disassembly notch." There's a tiny nub on the Slide Stop that catches on the slide until you hold the slide back just the right way, with a bit more pressure to hold back the slide, and then the Slide Stop will wiggle out easily.  On my gun at least, there's an actual slide lock position not mentioned in the manual that even without a magazine inserted appears to be where they want you to be able to remove the Slide Stop, but it won't come out no matter what.  I have to move my slide just past this point to disassemble.

Second difficulty I just learned how to solve, thanks to lklawson (thanks Kirk) was in putting the slide back onto the frame.  I had a heck of a time getting the Pederson block past the Front Rails of the frame.  Despite the Remington Video, I just couldn't figure out how to make that happen.  I resorted to holding the barrel back against the recoil spring completely , which is very difficult to accomplish what with the sharp edges near the feed ramp end, and the difficulty in holding the barrel all the way out against the spring.  With Kirk's note that I don't actually have to hold the slide and another review of the video ("wiggle the slide") I tried a few more times.  Turns out I can do it as long as I make sure the Pederson Block is set in the slide snugly and push a little harder and the slide goes back on.  No more figuring ways to hold the barrel against the spring pressure to reassemble!

Third difficulty I had was trying to load 7+1 or even load 7 at all with certain rounds.  I trimmed the bottom of the magazine followers on my magazines as suggested, and even resorted to the slingshot method of slide release for a bit of extra oomph, but certain hollowpoints were just nosediving on rounds 7 and 6.  I polished the feed ramp with a Dremel, and even polished the odd lips on the fronts of the magazines, but no luck.  I finally resorted to emailing Customer Service and they sent me a reply that they'll send 2 replacement magazines.  As noted elsewhere, the part number of these replacements (F305402) doesn't correspond to the one in the manual.  Could it be that Remington is addressing this issue after only a year?
Overall I would say the gun is easy to rack, and fun to shoot with a little bit of a learning curve (grasp the grip safety and keep the web of your hand away from the slide). 

Thanks for all the tips and learning.

Hi all,
Purchased a Gen II R51 in July due to its unique design and the $50 rebate.  The rebate coupled with the Big-box store's NRA discount in July resulted in a price point that I figure would not be so bad if I got a bad one,  So far, so good. 
Looking forward to learning all the tips and tricks from this community.
Thanks...