This windage drum is on an M1 rifle recently purchased.

I noticed that there were no "clicks" checking the windage movement on the sight base. It did not lock in place at all.  Tried tightening the center nut, but no effect.  So I removed the drum and found through searching, that it is a NM style drum that uses standard pinion threads.  Is that correct?    I could not find any references to an adapter plate of any kind that may lock into the notches as a normal drum would and adapt to the notches in the drum.

I did find some references about drilling the receiver for a detent ball.   How would this drum work?   I will have to buy a standard windage drum to replace this one.  Very curious.

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I did the detention ball modification on my NM M1, basically a hole in the receiver a spring goes in, and then a ball, the ball “locks” into those notches in the drum underside, spring holds the tension, enough you can turn it and get “clicks” like an AR15 rear drum. So yeah that won’t work for your standard base. Neat mod to do. Made the rear drum from a standard one by turning off the usual humps and then adding the notches like yours.

That looks like an SA Inc windage knob. I believe the ball detent mod was originally a USAF idea.

The way it works is that a small hole is drilled in the receiver and in many cases, a M14/Garand extractor spring and plunger or a spring and a ball bearing are inserted. The plunger or bearing indexes the windage knob in the notches cut or pressed into the windage knob.

If you need a normal windage knob, send me a PM.

The Springfield Armory M1A receivers have this hole in them for the ball detent and spring.  At least mine does anyway.  YMMV

As mentioned these work with the standard USGI base.  NM base requires a NM windage knob as they have a different thread pitch.  There's disk that could be installed under the elevation knob and would engage with the splines on the receiver that would give you NM clicks on a standard elevation knob.  Creedmoore Sports sells them.  This would make is so you don't need the hooded NM rear sight.

in the 1980s and early 90s it was fairly common to modify a GI sight with the  4 clicks per turn 1 MOA windage adjustment to a 8 click per turn 1/2 MOA adjustment by drilling the receiver for a detent ball and spring, and the using an NM windage knob with 8 detents in it, or turning off the lugs on the GI knob and then putting detents in it. Those detents had to be spaced exactly 45 degrees apart and be exactly the same depth and profile to get accurate 1/2 MOA clicks.

The pinion shaft for the sight was also milled to reduce the length of the boss on the elevation knob side to create room for a coil spring around the shaft between the ear on the receiver and the sight base.  This spring was then held in compression on the shaft and kept constant pressure on the sight base to keep it against the same thread on the windage knob.

The sight cover was also modified slightly to keep sufficient pressure on the sight riser, particularly at long ranges, and / or a shim fitted under the front of the sight base so it maintained constant contact with the sight cover to prevent the riser from rocking back and forth (and thus changing the elevation adjustment.

The riser itself was then fitted to the front sight block, usually by glass bedding to remove excess play.

1/2 MOA elevation was accomplished by either using the NM sight hood that rotated 180 degrees for a 1/2 MOA adjustment, or installing a thin plate between the sight ear and the elevation knob that had the teeth for 1 MOA adjustments on the ear side that meshed with those on the sight ear, and had 1/2 MOA teeth on the elevation knob side.

The end result was a very accurate and consistent 1/2 MOA adjustable sight.  There was still some hysteresis between elevation pinion and teeth on the riser, but you just had to remember that if you adjusted the riser down, to go two clicks lower than you wanted to end up, and step back up two clicks to ensure the hysteresis was removed.


Today, for an M1A you can just get an entire NM sight set for about $100.

reproduction commercial sets are around $100, USGI sets anywhere from $250-$300

What are the specs for the spring and ball bearing? they are missing from my rear sight.