My SD9's slide has a little movement from side to side. It does not "rattle" per se, but can be moved by hand a fraction of an inch. This is fairly normal, I think.
A round in the magazine, pushing up against the bottom of the slide, makes the side-to-side movement a little harder to detect, but it is still there.
The inside of the groves in the slide obviously have to be just a tad wider than the outside edges of the rails, which is what allows this side-to-side movement.
This little bit of slop allows the gun to function reliably during the full movement of the slide on the rails. It also allows some extra tolerance for expansion and contraction of the metal parts under different temperatures, such as when the metal expands on a hot day or after a number of rounds are fired, or when it contracts on a very cold day (I'd expect a little more side-to-side movement on a very cold day).
But if the slide were absolutely tight on the rails, then there could be too much friction or resistance to movement by the slide. And taking up that bit of slop would require very precise placement of the rails and very uniform milling of the grooves in the slide, both somewhat difficult and more expensive to achieve. Since the rails on the SD family are part of the Sear Housing Block and the Locking Block (which are held in place by pins inserted through the polymer grip frame) the molded frame would have to be made extremely precisely -- and we know that SD9/40 and SD9/40VE frames can vary a bit from sample to sample.
This little bit of slop occurs on a lot of different pistols. I just checked my Sig P239 and a 25 year old Mdl 5906; both have a little movement (the 5906, with full length rails, has the least).
Do others notice the same little bit of side-to-side movement in their SD9/40 and SD9/40 VE pistols? In other pistols?
