Had my new R51 out to the range today. 100 rounds of Rem UMC 115gr and 100 of WWB 115gr.
In the 1st hundred rounds, I had the "nosedive" issue while charging the gun multiple times, but I only had two FTF's while shooting, and one of those was when I loaded one round and then replaced it in the magazine (7+1) before firing, which others have said has been an issue). The second time, I thought that the 6th round felt like it "hung up" while I was loading the 7th, and sure enough it nosedived after the first shot. About half of the magazines from the first hundred rounds were loaded with only 6 rounds; when I did this, the gun functioned flawlessly.
Sometime during all of this shooting, I noticed something: when I was racking the slide, I was (without realizing it) pulling back until I felt the slide stop release, then riding it forward a tiny amount before letting go. As if I was making sure the stop had released before letting the slide go (though I wasn't doing it consciously).
For the second hundred, I loaded every mag with 7 rounds and racked it like I was trying to rip the slide off the back of the frame, letting go of the slide while still pulling backwards. (You know, like the instructor told you to do for the malf drills?) Lo and behold, every single mag loaded and fired perfectly.
I regret to say that I didn't even think about testing loading a mag in with the slide closed, as The_Guy reported. I'll try that and report back.
I can see why the Model 51 is so admired. The R51 (magazine issues aside) shoots like a dream. Very controllable, accurate, comfortable to shoot - I was having no trouble ripping out the centers of bullseye targets at 5 and 7 yards, and was doing respectably at 10. After shooting it, I'm definitely going to try and work out the kinks; it's totally worth it.