Question about why my zero adjustment changed

[drewQ22]

Ok. To preface this all I know this is not a problem really, but I was just wondering.

I originally had a Troy trx rail and have Daniel defense front and rear sights. When installing the Troy I bridged the upper receiver and rail w/ my larue mount for aimpoint t1. I didn't have to do much zeroing at all.

I changed rails to a DD ris ii and first mounted the rail and zeroed at 25 without much changing of windage and elev. This time I forgot to bridge the upper receiver and rail and I wanted my aimpoint one notch forward but the rail was preventing the mount from closing.

So, I loosened the rail and 'bridged' the upper and rail w/ aimpoint and then tightened the rail down. When I went to zero at 25 it took a lot of adjustment w/ the rear sight to get a decent zero.

I know having the rear aperture over quite a bit is pretty normal as long as there is enough room for adjustment.

My question is why when bridging the ris ii and upper did it change so much? Minute unevenness between the rail and upper receiver? Every rifle I've had hasn't had to have the aperture over to one side. I guess it is just a little bit of ocd.

[SteveL]

It's not considered a good idea to bridge the upper and the rail with any type of optic. Assuming you mean a FF rail, it's only attached on one end and is therefore susceptible to flex. This can affect POA/POI of any sighting system attached to it.

I think you answered your own question though. The fact that you could not bridge the gap with your optic without loosening everything back up first tells you that nothing is positioned the same as it was before. I recommend removing the optic, loosening the rail back up and seating it all back down properly. Mount your optic on the upper receiver as far forward as you like without bridging the gap and re-zero.

[drewQ22]

Yea I guess that makes sense about poi shift. I tend to overthink things I guess.

Here's pics where the first it's a notch back and the latter is when I had it as far forward as possible 'bridging' the gap
http://i1201.photobucket.com/albums/bb358/sabroso1/67B0CDAF-8085-4AE4-B901-7167FB94F340-2121-0000022811D2329F_zps769182c8.jpg

http://i1201.photobucket.com/albums/bb358/sabroso1/CC25FFBA-D545-4322-8161-B6D934CB0052-48720-0000051F05F93046_zpsbafceea1.jpg

[HeliPilot]

Metal expands when heated therefore flexing. The rail is directly touching the barrel nut which is directly touching the barrel thereby transferring heat through conduction at at much greater rate than through the upper receiver. Since the rail heats up faster than the upper this is where the flex occurs.

Mount the optic only to the upper and your zero will not shift when firing.