Fixing an M16A1 Rear Sight

posted Thursday, September 11th 2014 at 11:23 AM by


So I took my recently finished M16A1 clone to the range to be zeroed, and I could not for the life of me get the group to move left or right. After a bit of searching, I discovered the rear sight drum was broken. Note the hairline crack in the photo.

Crap. Now what?

First thing to do was find a replacement part. Midway USA has all the individual parts, as well as a bundle with everything you'd need. I ordered what I needed and 3 to 4 business days later was ready to go.

M16A1 rear sight repair

Here she is with all the parts. I like to wear safety glasses whenever springs are involved.

M16A1 unloaded

Safety is the first step. Make sure the rifle is unloaded by checking removing the magazine and checking the chamber for a round.

M16A1 rear sight parts

Here are all the parts laid out. I like to put everything on a high contrast background (e.g. white paper towel) so I don't lose anything and I can keep everything in one place. I ordered the full replacement part set, in case I needed another part or I lost the spring or the detent behind my workbench. As it turned out, I couldn've gotten away with only ordering the rear sight drum.


The next step is to remove the broken drum. Take a small punch and tap out the roll pin. The drum will come right off leaving the detent and spring free. Use care not to lose them.

M16A1 broken rear sight drum

Yup. There's your problem. The old drum had cracked off, leaving the dial to spin freely without actually moving the rear sight. Compare to what the new drum looks like in the photo.

M16A1 rear sight drum roll pin

Next, prep the new drum be inserting the roll pin. I use a pliers to make it easier. Make sure not to push it in so far that it protrudes into the center as it will interfere with the rear sight screw.

M16A1 rear sight drum install

With the roll pin in place, install the rear sight drum.

M16A1 rear sight drum install

Finish up by pressing the roll pin all the way through the hole in the screw. Again, a pliers makes this easier.

M16A1 return to battle sight zero

Lastly, return the aperture to battle sight zero by moving the aperture all the way to the left, then 17 clicks to the right.

If the battlesight zero has not been previously determined, place the M16A1 sights in the starting position.

  1. Set the front sight post so that the base of the post is flush with the top of the front sight post well, then go clockwise for 11 clicks.
  2. Center the rear sight aperture within the rear sight housing by moving the rear sight all the way to the left, then moving it 17 clicks to the right.


Adam Konieska M16A1

And finally, a little eye candy for this throwback Thursday. There you have it. A brand spankin' new M16A1 rear sight. I'll try and do a post later with a full range report next time I'm out.

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