What is it that you are calling "neck sizing"? Straight-walled cases can be said to have a "neck" (where the bullet is in contact with the brass), but I am not sure what you are calling the neck and which die you are using to "size" it.
Die # one sizes the fired case and deprimes it.
Die # two bells or flares the case mouth (is this what you are calling neck sizing?)
Die # three seats the bullet and (optionally) crimps the case mouth onto the bullet.
Die # four crimps the case mouth onto (into) the bullet's crimp groove.
It is worthy of note that at 600 fps, even in such a light gun as the LCR, you probably don't need any crimp at all. The "neck tension" is likely plenty to hold the bullet.
The case sizing (die #1) squishes the case to smaller than the bullet diameter. The case flaring (die #2) expands the case mouth, but does not stretch the case larger than the bullet's diameter very far down the case. 1/8" at most.
When you seat the bullet, you are pressing it in and the bullet stretches the brass some, but since brass is elastic it will grip the bullet tightly.
At this point, if you try to drive the bullet deeper or pull it out it will take about 40-50 pounds (I am told) to move it. This is without crimp.
At this point, if you run the cartridge into the seat/crimp die and just barely straighten the flare, you can fire theses rounds. Light loads don't need more bullet retention than this.
Now, when you run this round into the Lee FCD with the post-sizing ring you squeeze the brass cartridge AND the bullet. If it's a lead bullet, it doesn't spring back, but the brass does, loosening the tightness of the brass' grip on the bullet. This phenomenon exists with jacketed bullets, too, but not nearly to the extent it does with lead bullets. This is why many people eschew the Lee FCD in favor of other brands, of Lee FCDs with the post-sizing ring knocked out or using a combo seat-crimp die to crimp only or doing away with the 4th step altogether and seating and crimping in the third step.
I recommend the latter for those using a single stage press. It cuts the number of strokes you have to make by 25%.