Hope this is the right spot.  I am having an issue with my Poly 80 940 SC (Glock 26 size)

To get my irons to line up with the zeroed dot I had to drip the rear quite a bit to the left.  Have never had to do this with a Glock/ poly 80 and it just is odd to me.  

Build List
Poly 80 frame and parts (flat trigger)
Brownells RMR slide with window cuts
Bear creek Glock 26 barrel
Ameriglo black suppressor sights
Trigicon RMR

Gun shoots great but having rear drifted drives me crazy.  Especially if dot flickers or is not as bright (series 1 RMR01)

What should I do? change the barrel?  

Try a different barrel. If possible.

I concur as a means of a starting point.  It will tell you if its the barrel if the barrel shoots straight.  If it continues to shoot sideways, might be the slide.  Can also put the slide on another gun.  Only replacing the slide and keeping the frame/barrel if you are able too.

There have been reports of out of spec locking blocks causing issues with zeroing and accuracy.  If the locking block is not sitting squarely in the frame you can have the issue where pistol shoots right or left.   If the fit between the locking block and barrel is not consistently the same you’ll get erratic accuracy.  A Friend’s issued model 5904 had poor accuracy which was traced to an out of spec slide catch pin which impinges on the bottom of the barrel for lockup.   Essentially the same thing can happen with a Glock if the locking block and barrel lockup is out of spec or loose or inconsistent while in battery.

I have a P80 kit from MDX.  It shoots to the right.  Rear sight is all the way to the left to center the shots

@OP
Are you a right handed or left handed shooter?

Lefty

Put the gun in the hands of another shooter, preferably a right handed shooter, and see what your target looks like. If the POI shifts then evaluate your grip. If it stays the same, check the slide for indications that slide was milled slightly off center.