It's normal...........just the nature of the beast with some Rugers
Sometimes the guy drilling the holes for the front sight was a little "off" and they weren't perfectly tight. Some of my GP100's also have a front sight cutout that's a hair too big and the front sights wiggle a little. Just the way some of them are. Mass produced firearms always have some "variance".
A "rig job" repair would be to put some epoxy or bedding compound in the bottom of the front sight channel and re-install the sight. It should tighten it up. I have a Security Six that I plan to use for some "PPC lite" competition that I plan to do this to.
Out of my many Six series revolvers, about 4 or 5 of them have front sights that move a little front to back. Some are hardly used, one of them is like new but if I put my finger on the front sight blade, I can feel a back and forth "rocking". It shouldn't affect usable accuracy and in fact one of my best shooting Rugers, a Speed Six 4" has a sight that has a very slight fore and aft movement.
Side to side movement is a bigger concern and may require replacement, since this would indicate that the front sight took an impact and the hole is enlarged. I have a Security Six like this, but the front sight was obviously damaged.
The ONE advantage I can think of, of a S&W over the Ruger Six series is that the S&W's have tougher and more solid front sights.
On the other hand, the Ruger front sights are replaceable, whereas a damaged S&W front sight would require a big repair job.