It sounds like your problem may be the rails...FNSs (and if I recall correctly, FNXs) have changeable rails, so give that a peek and see if they just need replaced.Now you've got me worried. I have the FNX 9mm and it rattles. Hold the gun and push the slide from side to side. Mine is loose and rattles when I shake it, cocked or uncocked. All three of my 1911's are tight as a drum, and always hit what I aim at, not so with the FN. I'm lousy with it. I also have a Browning 9 by "FABRIQUE NATIONALE", made in Belgium and it's also tight and hits what I aim at, no such luck with the FNX 9. I thought it was me, but now I'm beginning to wonder. I may swap it for a Glock. But I've never even fired a Glock. My son is an ex deputy, who carried a 40 cal. and he loves it. Unfortunately, I've never had a chance to fire it. Hmmmmmmm!