I own one. It takes a good amount of force to lock the slide in place, without shells in the gun. When fired, I found this isn't a problem. As long as there is one round in the magazine, the slide locks forward more easily. As soon as the last round is stripped from the mag it becomes a little harder to lock.
Taking the gun apart and cleaning/lubing it helps as well.
Keep in mind that the way the FN system works is that pressure from the fired shell actually drives the slide rearward. It is the fastest pump I have ever fired. It hurts too
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I mounted a rail on it today, and placed a Tacpoint on it. This puts it in a bit better position for firing, and hopefully won't be as awkward to fire for me.
I plan on putting a Knoxx Spec Ops stock on it as soon as fundage allows.