Okay, so I had an off day at the range today.

I got to the range late due to a bunch of reasons, and as such, I was rushing to get my AR zeroed with my Acog, not really taking my time. I was zeroing at a distance of about 25 meters.

So I shoot a bunch of groups, make adjustments to the turrets until I'm dead on. Each time I finish my click adjustments, I whack the ACOG with the heel of my hand as per standard procedure.

Well right at the moment I was done, the range guy comes over to let me know that they are closing. I'm thinking I'm good to go. As I'm getting ready to put my AR away, I do a quick check on the knurled knobs of the ACOG and notice that they are way loose. Like I can turn them atleast a quarter turn before I begin to feel any resistance of them tightening. Now I'm paranoid. My ACOG felt solid every time I grasped or hit it while making adjustments. It never felt wobbly or loose. But only when I tried finger turning the mounting knobs did I notice that they were loose. Typically, I would always check them before shooting. But like I said, I got there in a rush and I was not being my normal methodical self.

I am definitely going back tomorrow to double check my zero, but how off do you guys think I will be?

I'm guessing 3 MOA.

That's alot. Even at 25 meters, that's like 3/4".

When I was shooting, most of my shots were touching easily, even with my rushed mindset. Could it really be that off once I shoot it tightened?

I think you'll be fine, but I would check it.

Blue loctite ALL your mount screws and zero it to shoot lower at 25 or you will hitting way way high at 100. Use a ballistics calculator.

I "zeroed" it to be dead on at 25 meters using the 300 meter mark in the ACOG. So the top of the line under the 100 and 200 aiming points. That should bring me to be zeroed properly enough using the markings in the ACOG. Tell me if I'm wrong.

He and I thought you were using the center aiming element, but you're correct.

If the mount was loose you need to start over again. I use a quarter to make sure the Colt style knobs are tight after moving the mount fully forward in it's picatinny rail slots.

IMO a 25 yard zero is only used so you will actually be on paper at distance. Final zeroing is done at full distance.

BTW - I never wacked an ACOG with my palm in my life and they hold zero just fine.

So I went back today to verify my zero. It was still spot on, luckily, even after really tightening it down.

So after confirming zero, I did some shooting using the ACOG lens cover closed. So it was completely occluded. Using both eyes open, shooting at targets 10 and 20 feet away, and then 25 "yards" away from the standing groucho position.

As expected, my shots printed about 3-4" below the tip of the chevron.  Next step is head out to the 100-500 meter range to confirm zero.

I have to agree with @borderpatrol

I did the angular math one time and it does not take much to be off multipe inches down range...

If I were you, start over, will the only way you will know you are true

1)  Turning the knobs "a quarter-turn past hand tight with a quarter" or similar methods aren't really the best.  You can torque those knobs down.  Use a Wheeler FAT Wrench and go to about 30 in-lbs.  Use a very wide slotted tip that fits the knobs correctly.  Even with that base, you can still have forward and back movement.  Just pushing it forward and tightening down doesn't mean it won't slide back when the bolt slams forward.  It's a semi-auto rifle so this can happen.  You can insert and glue a piece of rubber in the slot with the mount to make sure it can never move back and forth.  
2)  Better yet, ditch the TA51 base and get a Geissele mount.  I torque mine down to 50 in-lbs but you can go all the way to 65.  It's so much torque that not only will it not come unscrewed, it also won't move back and forth between the slots.  Also the Geissele will fit the rail better so even if its comes loose it can't slide much at all.  
3)  Don't zero at 25.  Zero at at least 50 or 100.
4)  Occluding the lens and pretending it's a red dot may cause a substantial horizontal difference between point of impact and point of aim due to something called phoria.

What is the reason for "whacking it with the heel of your hand"???

"TM 9-1240-416-13&P
1-17
ADJUSTMENT PROCEDURES
The M150 is internally adjustable. The adjusters need only position the internal roof prism. For this
reason, a light tap on the adjusters, after an adjustment has been made, will ensure proper seating of
the internal mechanism and allow for an accurate zero. If a light tap to the adjuster is not applied, the
first round fired may be inaccurate."

Read the F'ing manual boot!!!!!!!!!

I'd verify and get a new mount

This is the only way I would get my ACOGs to seat....and belive me, it did make a difference and resulted in a POI shift.

What @DefenderAO and @bfoosh06 said

For what I use these rifles for, I do not need a removable optic mount,  The knurled ACOG mount is designed to be removed in the field.  Over the years, I have gotten rid of my "lever mounts" except for my EOTech built in levers, and went to a mount that requires a dedicated torque....I have swithed all the mounts over to Geissele....65 in-lbs torque.

If you are dead on @ 25 with a 2.6" sight height you will be 6" high @ 100 and 10" high @ 200 and zeroed @ around 400

Load Number
XM855FL
XM855BK420 AC1
XM855 LPC120
XM855BK150
Caliber5.56x45mm
Bullet StyleFull Metal Jacket Boat-TailBullet Weight (grains)62 grs
G1 Ballistic Coefficient.349Muzzle Velocity3,020 fps
Sight Height2.6 inZero Range25 yd
Temperature59 °FWind Speed10 mph
Altitude0 feetMax Range400 yd
RANGE (YD)DROP (IN)WIND DRIFT (IN)VELOCITY (FPS)ENERGY (FT-LB)
0  -2.6 0.03 3020 1255
25  0.0 0.0 2951 1198
50  2.4 0.2 2882 1144
75  4.4 0.5 2815 1091
100 6.3 0.8 27491 040
125 7.8 1.3 2683 991
150 9.0 2.0 2619 944
175 9.9 2.7 2556 899
200 10.5 3.6 2493 856
225 10.7 4.5 2431 814
250 10.6 5.6 2370 774
275 10.1 6.9 2311 735
300 9.1 8.3 2252 698
325 7.7 9.9 219 3662
350 5.9 11.7 2136 628
375 3.6 13.6 2079 595
400 0.9 15.6 2024 564

I zero 1" below line of sight @ 25 to be more near a 50/200, unless the optic suggests otherwise for BDC.  Then run the numbers for a 100 yard zero through a ballistic calsulator to see where you need to be @ 25.

B/C