POA/POI question - something's not right..

[polydeuces]

Need some input please.

When shooting @ ~15 yds., POI (lateral) is off by about 1.5" to the left, with rear iron sight already adjusted as far as it goes.
Noticed it when running drills, then verified from bench.

Front sight is a YHM gas-block/flip up.
Rear sight is Magpul BUS, mounted on upper.
DD 16" CHF upper w DD Omega free float, should not be not an issue, has nothing mounted on it.

My initial thought is perhaps the FS/gasblok is off-kilter? It look pretty straight to me, and wouldn't that affect the gas-system? Other than that it's running flawless.

Something obviously is seriously off - cant figure out what.:confused:

Thanks.

[spamsammich]

It doesn't take much canting in the front sight to put it off more than your BUIS can adjust for while still leaving the gas tube straight enough to run. Set your upper into a vice and level it out with a 4 foot level, hang a plumb bob in front of the upper and check that gas block for plumb.

Another way of seeing how much tilt would be to wipe off the mouth of your gas key, mark it up with sharpie on the inside and then insert the BCG back into the upper minus the charging handle and work it back and forth several times. Then look at the gas key to see where it is contacting the gas tube, then adjust the gas block from there to minimize the rubbing. Once you confirm that the front sight is plumb, I would mark the gas key again and repeat the testing procedure. If it still rubs in one spot you may need to reach into the upper and bend the gas tube a little.

[polydeuces]

Looks like I'll be taking the gasblock/flip-up sight off and re-mount, this time using a level. Initially mounted it using visual "reckoning", maybe not a good idea.....:o

Now I'm wondering, would it be possible to somehow "knock" it out of whack during use? Not that i think that's what happened, It's on there REALLY tight.

Don't recall slamming it against anything - not that I'm particularly careful with it. That would be really bad, and a good reason to rethink this type of set-up.