I just mounted an Aimpoint Comp ML2 on my Bushmaster A3 Carbine.  I'm using an A.R.M.S. 22M68 with spacer.  At sight in, I had to use all the down adjust to zero it at 25 yards.  I've only got 3 clicks left to bottom out the adjust screw.  I counted 300+ clicks of range on the screw.  Seems I could shim between the rear of the spacer and the mount.  Anyone else have this problem?  The rifle is new and I haven't tried the open sights.  When I went to the range I left the handle/rear sight at home. Also, what's a good device to replace the front sight and gas block in order to get the sight out of view?  Thanks in advance for suggestions.

May wish to try at 50/100 years also to get an idea of how far off you are.  At that time you could try the iron sights and possible another scope / red dot from a friend.

Before I started to shim, would first take the mount / rings / riser apart and check the contact surfaces for any lumps/bumps/material (possible carefully clean the contacting surfaces).

Then when I was putting all the mounts back together, would wiggle each matting surface a fraction of a mm while putting toghter - to get the best contact (along the lines of putting in a screw which is slightly misalignment).

In connection with front sights, most of us end up with some type flip up rear sight (ARMS #40 is nice or the Kights 600 meter sight is you are into longer ranges ;-) and keep the front sight.  Would recommend doing a search on AR15.com for "front sight" and setting the time frame to max (30 days most likely).

[url]http://www.ar15.com/forums/topic.html?b=3&f=118&t=160978&w=searchPop[/url]

[url]http://www.ar15.com/forums/topic.html?b=3&f=18&t=160382&w=searchPop[/url]

Just to verify that it is not in any ring or mount, try reversing the optic in the ring and put the ring attachment on the opposite side of the weapon and in the same nothch. Also make sure there is no nick in the dovetail rail that your attaching to. You will prob. find it is not in the ring, but something else.
Good shootin, Jack

Thanks for the replys.  As always, it's the simple things that get missed.  I'm going to disassemble everything and clean the mating surfaces.  Retaining the open sights is also a good idea.