ACOG adjustment test

[StainlessSlide]

The ACOG 4x32 BACs are my primary sight of choice. Naturally I am interested in some technical issues which they raise. I’ve long been wanting to explore their adjustment characteristics using an improvised optical bench.

Optical benches are quite expensive, so I thought I’d make my own crude one. I started with a carbine length upper assembly which I have disabled for safe dry firing. It has a rod expoxied in the chamber, and several other fail-safes. I epoxied the spud of a Sweeny boresighter into the muzzle end. The boresighter acts as my collimator. It projects a grid into the scope. The grid is graduated in 4 MOA intervals.

On the upper I mounted a RCOA4, which had been torqued, loctited and marked into a LaRue ACOG mount. The LaRue levers were down tight. I lightly clamped the upper into a bench vise, tightening on the MI drop-in railed handguard (not free-floating).

It was not easy to get the boresighter grid rotated into alignment with the reticle, since the ACOG has some edge distortion. I aligned it to match the reticle’s horizontal direction to within half a grid square over its range of 20 grid squares.

My testing protocol was to move the adjustments through a 30 click (15 MOA) square. I couldn’t tell where mechanical zero was, but I started at the point where the ACOG had been sighted in last. Since the ACOG needs to be tapped after adjustment, I used the following protocol:

Taps - 10 taps on each adjustment knob cover with an empty PMAG. I used the front of the floorplate. I tapped about as hard as I would in tapping a thumbtack into corkboard.

Harder Taps - The same schedule, but I tapped about like I was driving a thumbtack into soft pine.



Here are my results - adjustment directions are the bullet impact direction, not the reticle movement direction.

Since the boresighter was coarsely graduated in 4 MOA increments, these results are only good to +- ½ MOA

STARTING POSITION - Chevron tip was Down .1 grid square (.4 MOA) Left 0 grid squares (0 MOA)
After tapping - No change

UP 30 CLICKS (15 MOA) – Tip was Down 3.9 (13.7 MOA) Left .2 (.7 MOA)
After tapping - no change

LEFT 30 CLICKS (15 MOA) - D 4.1 (14.4 MOA) R 2.8 (9.8 MOA)
After tapping - no change
After harder tapping - D 4.1 (14.4 MOA) R 3.0 (10.6 MOA)

DOWN 30 CLICKS (15 MOA) - D .6 (2.1 MOA) R 3.8 (13.4 MOA)
After tapping - D .7 (2.5 MOA) R 3.8 (13.4 MOA)


RIGHT 30 CLICKS (15 MOA) - D .1 (.4 MOA) R .2 (.7 MOA)
After tapping - No Change
After hard tapping - No Change


It looks like everything turned out as well as could be expected for an ACOG, except that the left adjustment was 5.2 MOA short over 15 MOA. I looked through the ACOG while making this adjustment, and the movement slowed down over the last 7 clicks or so (it didn’t stop dead, as if I had reached the mechanical limit of adjustment). A little of this shortfall came back with the harder tapping, and most of it came back after the down adjustment. The left adjustment of the scope involves backing out the adjustment screw, not screwing it in.

One thing that surprised me was that firm finger pressure on the muzzle could move the point of aim on the boresighter by 4 MOA (remember that the barrel was clamped over its non-free floating hand guard). Firm finger pressure on the rear of the upper (a little aft of the takedown pin hole) could do the same.

[Eurodriver]

How is this relevant given that the ACOG should be zeroed once and used in accordance with the BDC thereafter?

[StainlessSlide]

Better understanding of the mechanical systems we use seems like an obvious good thing.

[Iraqgunz]

MOVED THE CORRECT AREA.

Better understanding of the mechanical systems we use seems like an obvious good thing.

[StainlessSlide]

I've been playing with this system and have a few more results.

It looks like the total range of adjustment of this reticle is about 80 clicks in the elevation direction and 72 in the windage. I started my square path roughly in the center.

There are places in the reticle windage adjustment where the change in the aim point stops and actually reverses direction for about 2MOA. There is a slight elevation change which occurs at the same time, so the chevron tip moves in a 2x1 MOA loop. This happens when moving through the same area in both directions. Tapping didn't have a corrective effect. Sometime like this was mentioned in another thread, but it's different seeing it with my own eyes.

In my reticle, this anomalous point was 12-16 MOA from mechanical center. Everywhere else the adjustment was smooth and reasonably well calibrated.

This might explain stories about some users "chasing zero", if their zero point was within one of these areas.

My takeaway from this is not to be surprised if unusual things happen during zeroing. Just keep making the indicated adjustment, even if it results in the group moving more or less than expected, or even moving the wrong way. And (of course) tap after each adjustment. Eventually the group will resume moving in the right direction.

I tried to calibrate the boresighter grid by looking at it with an SWFA SuperS 10x fixed scope, and found that the grid squares were not 4 MOA as I thought, or 4" at 100 yds as marked, but were very close to 1 milliradian (1.02 milliradians by the SWFA reticle). I have changed the numbers in the OP.

Addenda:

For those interested in what an ACOG looks like inside, here is the ACOG patent

http://pdfpiw.uspto.gov/.piw?PageNum...3DPN%2F4806007


And here is a promotional drawing:

0facc740cc51.jpg

[Hkbeltfed]

Can you explain why it needs to be tapped after adjustment? I ask as I recently purchased a mini acog, my first.

[infidelcrusader]

Spring reset

[StainlessSlide]

Yes, when a scope is adjusted through a few MOA, a tube inside the scope which contains lenses (or in this case a prism) pivots by a few thousandths of an inch (or less). This is caused by an adjusting screw pushing this internal tube against an opposite spring (and plunger), or backing out and letting the spring push the tube. As you can see from the drawing above, the windage spring is contained in the little boss on the left side of the scope, opposite the windage screw.

Or the scale of thousandths of an inch, the spring or plunger can stick, or maybe the bearing which allows the tube to pivot can stick (the clearances have to be pretty small). Impacts on the scope should overcome the stickiness, and allow the tube to reach its equilibrium.

The ACOG that I tested above was very smooth and regular in its adjustments, except for a few trouble areas where tapping made a small difference. Of course, different bearings, springs, and plungers could behave differently.