Smacking an ACOG after adjustment - is it really necessary?

[Eurodriver]

I have spent a lot of time using the ACOG, and teaching Marines to use the ACOG. I have been told from day one that in order to properly zero an ACOG you must smack the optic after any adjustment to "reset the prism". So naturally, I told everyone they must smack it as well.

Yesterday, I put this to the test by shooting two current USMC inventory (Bible verse scratched off and everything) TA31s. One was a TA31RCOM4 and the other an A4. The procedure was to fire both rifles from a rest at 100 yards and perform a box drill on a target. Each box was 12" apart, corresponding to 36 clicks of windage and elevation. I stayed behind the rifle holding it steady and looking through the optic while a buddy made the adjustments. I could physically see the reticle moving with the clicks and ending exactly where it was supposed to. After resetting on the original target, I found absolutely no reason to smack the ACOG as the rounds impacted exactly where they should have.

I am beginning to wonder if there is something going on internally that I am unaware of at the user level, or I spent several years wasting people's time on the KD range...

Does anyone have any information on this? How did this get started?

[markm]

How did this get started?

Strikes me as a classic USMC myth. I've never even heard of it.

[nimdabew]

Grunts need a reason to hit things.

[TAZ]

People used to smack the adjustments of old / cheap scopes to get the pins to settle into place and lock the turret if you were stuck between clicks and didn't know it due to the fact that the scope had not effin feedback. People also used to tap scopes to seat the erector mechanism in place. Weak springs, poor manufacturing... could lead to an erector lagging behind the knobs or staying out till after your first shot. Same reason why people often say to adjust your scope in one direction only.

Pretty much not needed on quality modern designs and manufacturing. My Aimpoint, IOR or EOTech track well any way I turn the knobs and don't need the tapping to seat anything. My craptastic Millett DMS would require the tapping to seat the turrets. I could watch the elevation knob jump half a click then stay put. Needless to say that POS is long gone.

[Failure2Stop]

Not always needed with the COG, but often enough that its worth it to make sure your stacked failed adjustments don't send the shooter on a wild goose chase.
I have witnessed failures of the optics to adjust.
My experience is based on RCOs acquired between 2005 and 2009.
It is possible that then new models were susceptible to the issue, and possibly with use the stickiness clears up, or that the design has been corrected, but I can say without a doubt that it was an issue.

[sevenhelmet]

Can you explain what you mean by "stacked failed adjustments"? Either the prism moves or it doesn't, but if it doesn't would that not indicate something was broken internally?

I have never heard of this before today.

[Failure2Stop]

Can you explain what you mean by "stacked failed adjustments"? Either the prism moves or it doesn't, but if it doesn't would that not indicate something was broken internally?

I have never heard of this before today.

Told to me by employee of Trijicon around 2005:

The ACOG (at least in the version that I am discussing) adjusts POA by moving a prism against springs. When moving it is (was?) possible for the prism to hang up when it was relying on spring pressure to keep the prism tight against the adjustment screw contact.

If the prism hangs-up during adjustment, the full value of the adjustment will not be applied, and after a few rounds, it will generally jump into place. It is also possible (as per the explanation, and seen in use) for the adjustment to fail to fully apply, which leads to an over-correction on the next adjustment, and if a following adjustment is made, when the prism finally pops free, the group will have drastically shifted.

I have not taken a TA-31 apart, I have not seen drawings of the internal components of the TA-31, and I do not know if any changes have been made to the assembly or parts in the time between the years specified and current production. I do know, for a fact, that I have seen TA-31 RCOs that showed identical POI of the first group fired with the next 1 to 5 rounds, and eventually the group would shift to the intended degree of adjustment either after a few rounds or after being firmly tapped (straight against the elevation and windage caps).

[Onyx Z]

I grew up with my Dad doing this to various scopes on hunting rifles. I still do it by habit on all scopes... I've never actually tested it though.

[scoutfsu99]

Strikes me as a classic USMC myth. I've never even heard of it.

I've seen it stated in the Triji manuals to do it. Helps seat the prism apparently.

[GTF425]

I do that with the M145 because the external adjustments are notorious for collecting dirt and not adjusting until you knock the debris out.

As for the ACOG, I give mine a few taps between making adjustments and firing another group.