I have a Sig 556 rifle that comes with the rear rotary diopter sight. The issue I'm having is that the drum (the part with the range markings on it) rotates as the rifle recoils, especially at the 100 yd setting. This results in the shots stringing significantly right to left as the rifle is fired.

It seems like the sight has no detent for the settings anymore. There is a set screw on top of the drum, but tightening it seems to have no effect. Is there something I can do to keep the sight in place? I'm not sure how to take it apart, or even if I could, what would need to be fixed.

I'd buy a new one if I could but they seem to have gone out of production years ago. Thanks in advance for any input you may provide.

I copied this:

For those having the "loose drum" issue...or those not happy with how easily the rear drum rotates accidentally, here's a simple fix. Pull the sight off the rifle. Use a punch and remove the roll pin holding the windage adjustment screw in place. Push the detent on the retainer on the right side of the sight and pull the sight apart. Use a hex wrench and remove the screw on the top of the drum. No separate the drum from the elevation adjusting wheel. Now take a spring out of the kit you'll have purchased from home depot (see link below). Pick a similar length but stronger springs and replace the original with that. Now reassemble the sight in reverse order. The detents will lock nice and solid now and this gives the sight a much higher quality feel. It locks like a HK or Swiss Sig rear sight from one setting to the next and will not accidentally rotate while the rifle is slung. We shouldn't have to do this to a factory part but unfortunately Sig decided to use a weak spring will very little tension in the rear sight. Hope this helps some of you and makes your rear diopter sight a usable part once again instead of an airsoft quality part...

http://www.homedepot.com/p/Eve...2045461#.UlDcBZ3n9Ms

I will give that a try. Thanks.
Home Depot spring set doesn't seem available, but I'll check around for similar springs once I see what the old spring is like.

Wolff Gunsprings sells variety packs of gun springs that you can cut to length yourself. They are pretty handy to have on-hand for projects like this.

https://www.gunsprings.com/GEN...E/cID4/mID131/dID364

Got a pic of the sight? I have a couple spares I can take a look at what to replace