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.
