I have a Millett SP red dot sight mounted to a S&W MP15-22.  For the last three ddays, I've been shooting paper targets at 100 yards after work in the evening. I've noticed that it does not seem to be holding zero shooting the same target at the same distance, when I begin shooting the next day. My group centers are about 3 inches off each time. I understand that nothing is completely absolute, but what is an acceptable amount for a red dot to go off its zero?

Don't know about the Millett but could be some eye position or paralax issue.

My Aimpoint always holds zero even when used on the FAL.

For what you spend on an Aimpoint it better hold zero!!!

I would say there is no acceptable amount for a red dot to go off its zero. That being said I have the Millet Sp red dot on one of my Ruger super redhawk revolvers in  .44 mag.
The red dot sight has held up to over 500 rounds so far with out any problems with zero. Battery life is another story, It burns them up quickly. 100 yards is hard shot for me with that gun, i would be really happy with 3'' groups at 100.

My 10 shot group is is running about 3 inches with Federal bulk ammo. Its the centers of the group that seems to be moving.

Not acceptable in my book.  I had a similar issue with an EoTech.  Now I run an Aimpoint and it holds zero, best move I've made.

There is no way you had an eotech that "looses" zero...you are making this up or just didn't know what you were talking about. Sorry, but I have to call you out.

My 2 Eotech's & 2 Bushnell Holosights never loose zero. Neither does my Colt 3X carry handle scope.

wow, keep the call outs in GD, if the man says it lost zero then it probably did. my wifes cousin doesn't like the EOtech because he had one blinking out in Iraq. he's in marine force recon, want to call him a liar? even high end optics can have issues, if it's man made, it can break. Aimpoints seem to have the least amount of complaints of all the red dots i've seen posted on here, but still from time to time they have issues.

Not sure if you're being sarcastic or not, but what I wrote is truth.  It was a new in box 512, I did an initial zero, and it held fine for a few months during 3 or 4 outings.  Then I put the rifle away for about 6 months.  When I took it out, the batteries were dead (big shocker there).  After replacing the batteries, the POI was about 5"-6" off (high and left) at 50 yards.  I re-zeroed, put away for another few months, and the next time out it was a few inches high and left again.  I replaced the sight with an Aimpoint Comp M3L and couldn't be happier.  I do really like the EoTech reticle, maybe I'll own another some day, once they get their shit together.

OP, sorry for wandering off topic.

Did it occur to you it was a bad site? Did you contact Eotech to have them look at it. A brand new sight would have been under warranty.
As someone stated anyone can make a bad 1.
Your story is not the norm.

Actually, I did contact EoTech, and their customer service was so incredibly shitty it pisses me off just thinking about now.  They basically wrote me off as being a poor shooter and that there was no way the sight could have such issues.  I do understand that my story is not the norm, but it was applicable to this thread so i mentioned it.