I was blasting away with my 223, which is a real good shooter, this morning. Groups weren't as good as usual and were mostly horizontal. I took that to mean 'user error', so I went back into the air conditioning to ponder on the problem. What I finally decided was that it was me (a terrible thought) or maybe it could be a parallax issue (easier for me to live with). Walked back out to the bench and had a look at the crosshairs while moving my head around a bit. Hmmm, mucho crosshairs movement. I adjusted the scope AO till the crosshairs quit moving, which had the scope AO set on infinity. Well, I can live with that. Groups tightened right up.

The scope is an old Leupold VariX III from the early 90's, but was refurbed by Leupold a year or so ago.

Got more shooting and reloading to do today, so I'll see how it goes.

I checked a few other scopes, and they seem fine.

Yes, I've had several customer rifles brought to me that had parallax issues like this. Generally speaking, the higher the magnification, the more noticeable parallax becomes.

To combat the parallax problem, I move my head back (for more eye relief) on the stock to where the scope sight picture begins to tunnel to a smaller sight picture. I will center up the smaller tunneled sight picture in the center of the cross hairs. This allows your head, or eye, to maintain the same location each shot. It's a band aid fix, and doesn't solve the issue. It just allows you to minimize any parallax issue.

I do the same thing when using a non-AO scope. I think it's a pretty effective technique.

Good that you figured it out!

I tried what Chad and RiverRider suggested. Worked nicely, and so I decided to test whether or not my chosen H335/40 gr BT load was really the best possible load for my rifle. I had a lot of reloading of 223 to do, so now was a good time to decide if that load was the best or not. I found a better load, or call it a better node, with a higher powder charge. Put 4 in a ragged hole and put the "dreaded 5th shot" a 1/4 inch out. I hate that. It was me, not the rifle. I knew it when I pulled the trigger. Maybe I should see a Shrink about that 5th shot.

Anyway, the new load prints exactly where the old load did. Surprising, but pleasing. I do think that the reduction in parallax error is what allowed me to find that new load so easily.

And, just to mention it, I finally had to clean the barrel prior to the load work ups. Major amount of carbon fouling, but practically no copper fouling. After cleaning, groups shrank noticeably, even before I found the new best load. Probably had 100 rounds or so through it since it was cleaned last, though I don't count rounds fired.

I'll be sitting next to you in the waiting room!

Achieving group shooter peace is overcoming the fifth shot self psych-out.