I am a big fan of the Trijicon ACOG ever since I used one in Iraq. Since 2006 I have owned over a dozen ACOGs, buying them cheap, and later selling them for a profit.
When you are looking to buy a used ACOG, this is what I recommend you do:
1. First, find and check the serial number:
Serial numbers are always on the top of the ACOG . Often you can also find the serial number on the left side of the ACOG.
I highly recommend calling Trijicon and finding out if the optic you are looking to buy is stolen government property. You do not want to spend your money on stolen property. Also you can find out how old an optic is. For example this optic was made in 2007 and was sold to Optics Planet.
2. Check the lenses for damage. ACOGs can get run hard, so clean the lenses and check them for damage and scratches. While I could not get them to show up on camera, this particular ACOG has some nasty scratches and damage to the ocular lens. Generally I would recommend against buying ACOGs with damaged lenses unless you are getting a very good deal.
3. Check the windage and elevation adjusters for damage.
I have seen used ACOGs where the adjusters were very badly damaged from people using the wrong tools to adjust them. Also I have seen the brass threads damaged for the protective caps. Check to see if the o-ring is present and undamaged.
4. Check the mount.
Once you have bought a used ACOG, check the rail mount if yours comes with one. ACOG’s can be used with out an adapter to mount directly to a M16 carry handle. Then there are a variety of adapters allowing ACOGs to be mounted to rails. Normally you will see the Trijicon TA51 thumbscrew mount attached to the ACOG by two screws. Check that the mount is installed correctly and that the screws are tight. I have purchased used ACOGs where the mount was on loose, and have seen people put so much locktite on the screws and mounts that it did not fit correctly together.
If your ACOG came with a TA51 thumbscrew mount, and had the knobs on the left side of the optic, this would be a good time to flip it around and have the thumbscrews on the right side of the optic. This keeps the thumbscrews out of the way when you operate your AR’s charging handle.
5. Check the iron sights. A couple of models of ACOG come with iron sights. If you purchased a TA01NSN or an ACOG ECOS, your optic has a set of secondary sights.
These irons are suppose to be factory zero’d for 50m. I have often found them to be way off. Regardless if your ACOG is new or used, check the zero on the auxiliary iron sights. Windage adjustment can be made by drifting the front sight.
The most important thing prior to buying a Trijicon ACOG is to make sure that is not stolen government property. After that, checking the lenses, adjusters, and mount will help prevent issues that could have been caused by a previous owner.