Handy, nice to hear from you over here at The HighRoad. I recall a number of posts from you at TFL. Like you, I have an interest in the various operating systems of different firearms. You've answered some questions for me in the past, I believe. Maybe I can return the favor here.
As for the FN 5-7, I went through this same pursuit a few months back. Like you, I just wanted to know how the 5-7 works, given that FN advertises the system as a unique form of delayed-blowback.
Anyway, after much searching, I managed to find what I'm pretty sure is the US patent for the FN's mechanism.
With all due respect to GooseGestapo and unless my information is wrong, the 5-7 appears to work nothing like the CZ-52. No locking rollers. As you know, the CZ-52 is a pure recoil operated mechanism whereby the locking of the slide and barrel for a short distance are accomplished by rollers.
The 5-7 mechanism does appear to be unique. It sits somewhere between a recoil mechanism and a delayed-blowback mechanism. Like recoil, the 5-7's barrel moves during cycling of the mechanism, but unlike recoil the barrel and slide are never really "locked" together. Like delayed blowback, the 5-7's slide is heal closed only by the recoil spring and a mechanical disadvantage holding the barrel against the slide. Again, though, unlike traditional delayed-blowback weapons, the 5-7's barrel is not fixed.
Okay, enough prelude. Here is how I understand the 5-7 to work. The barrel and slide are "linked" by a lever. Before the slide can travel rearward, the lever must first rotate towards the shooter thereby releasing the slide. However, the lever is held rotated forward by the barrel. When the 5-7 is fired, the bullet catches the rifling in the barrel. The friction between the bullet and rifling in the barrel is enough to create a force which wants to push the barrel forward. (Think Savage pistol, where the bullet's friction with the rifling prevents the barrel from rotating to release the slide). The barrel being held forward means that the lever cannot rotate reawards, which means, in turn, the slide cannot be released rearward. Thus, the slide is delayed from opening the breach until the bullet leaves the barrel. Only once the bullet leaves the barrel does the friction holding the barrel forward cease.
It's a delicate balance, as most delayed-blowback mechanisms are. The mechnical lever delay is setup so the friction of the bullet traveling down the barrel is sufficient to hold the breach closed. Once the bullet is gone and this force is gone, the remaining case pressure is enough to open the slide and cycle the action.
A picture (or diagram) is worth a thousand words. Unfortunately, I don't have either that I am able to post.
This is my understanding of the 5-7. If I am wrong, someone please let me know.