DRPMS LR-308 Rear Iron Sights - Explanation sought

[kerrmudgeon]

Does anyone know why the DPMS LR-308 rear iron sights mounted on the detachable carry handle have two knobs for elevation on the same shaft? They look as though they should be fused, but the lower knob is capable of rotating [respecting the detent] independently of the 'larger' knob with elevation markings such as '6/3'. Reiterating, the lower knob has no numerical markings on it that I'm aware of.

This seems counter-intuitive and somewhat useless since the lower unmarked knob is capable of raising the rear sight thereby rendering the numeric markings on the upper knob meaningless. It can't be for one-time tuning, as the front sight post should be raised or lowered for that.

DPMS's manual is very light on details regarding the LR-308 sights, and while I assume they were designed to be functionally identical to AR-15/M16 sights, I wasn't aware AR-15s had *two* elevation knobs.

Anyone care to comment? Am I missing something?

[Reddbecca]

A picture would be a big help.

[Absintheur]

One is for elevation, the other should be for windage which is how the AR's are set up. The side knob is the windage with clockwise moving the point of impact to the right, the flat horizontal knob is elevation, clockwise moves POI up.

[kerrmudgeon]

Originally Posted by Reddbecca

A picture would be a big help.

I completely agree. Here is my attempt at an illustration. The actual rear sight is the detachable carry handle, but this GIS result may help clarify my question:



I've identified the windage knob as well as the elevation 'assembly.' The detents in my rifle are particularly stiff, and I was only able to make the 'lower elevation knob' rotate [after the help of vice grips - not a great idea in the vicinity of aluminum in case anyone was wondering]. The lower elevation knob is free to rotate independently of the upper elevation knob which I haven't managed to get moving past its detent.

The lower elevation knob is unmarked. The upper elevation knob is marked with 6/3 (indicating 2.6 inches above bore, 300 m zero) as well as other markings for positioning iron sights at ridiculous ranges (the 5 in the image suggests 500 meters).

The zeroing procedure says "set rear sights to 6/3, click up once, then adjust front sight post to achieve zero at 25 m. Click down, and weapon is now zeroed at 300 m. Use other settings on the rear sight for longer ranges."

This is all well and good, but if the lower elevation knob without markings can move the rear sights, what good are the numeric markings? It seems like something is amiss, or that I don't understand.

DPMS's manual isn't particularly clear on this point. Their AR-15 schematic clearly shows these parts (#80 and #79 in the image).

[tunder]

Are you sure the upper and knobs should turn independently?

[kerrmudgeon]

I am certain the upper and lower knobs move independently.

I've only been able to make the lower knob turn, and that took applications of tools I'd rather not be using. The upper knob seems to have some mechanical play about the threaded vertical, but I haven't made it turn an actual 'click'. I'm assuming the detents are just factory stiff.

An alternative explanation maybe there is a pin somewhere there shouldn't be, and that the two knobs should be mechanically coupled such that they turn together. That would make lots of sense. It seems maybe that is the intent (#83 seems to be a screw that could serve that purpose) but I've yet to see a definitive assembly guide.

Thanks for all the responses.

[Rowlock]

I'm wondering whether one dial is some kind of "locking nut" for the other one. It doesn't really sound like it from your description but I'm not sure.

Can you slacken it off and see if the other one becomes easier to move?

[Absintheur]

I under stand the problem now that I have seen your pic and read the explanation of how you tried to adjust. You have a broken sight. The elevation wheel may be made from two parts but the two halves are not meant to turn independently of each other.