OK, here the scoop with my DPMS detachable rear sights. I have three of them, and they are this model:
However, from your comments and photos I have seen, I believe the DPMS detachable carry handles have the same problem.
The ones I have (which are very different ages) were made by DPMS using the incorrect A2 sight base, which has a coarser elevation thread pitch and which is intended for use with fixed carry handle uppers and a 3/8 elevation index. However, for some reason known only to DPMS, they use the 6/3 index like on regular carry handles. This means that the elevation adjustments are completely meaningless and, with the coarser thread, the rear sight cannot go all the way up to 600M.
I fixed mine with the following modifications:
1) Drive out the roll pin below the elevation drum, then crank the elevation drum in the direction of increasing range (i.e. sight base gets raised) until the rear sight assembly can be removed from the sight body (remember to catch the ball bearings so they don't dissappear behind the sofa). Now you will see a slot in the bottom of the rear sight base, near the end of the elevation thread; metal at the lower end of this slot is what is stopping the elevation from going beyond 500M. You need to open up the bottom of this slot (extend it to the end of the thread) using a dremel, so that the roll pin no longer prevents the sight from reaching maximum elevation.
2) While the rear sight is disassembled, change the elevation index from the standard 3/6 version to an A2 3/8 version (available from Brownells). This will match the index to the thread pitch of the sight base. As a side benefit, it gives you adjustment out to 800M instead of 600M like any other carry handle.
3) Reassemble the sight and reinstall the roll pin.
Of course, now the roll pin will no longer prevent the rear sight from unscrewing, so you have to be careful not to go too far when adjusting elevation, but you have a good deal of leeway. The sight will work like a regular A2 sight; for the IBZ with a 20" barrel, you go -2 clicks for 200M and the rest of the elevation markings should be correct.
I wish I had taken some photos the last time I did this. If you have any questions, send me an EM or IM.