I have a Bushmaster M4gery and I'm having problems zeroing in on 50 yards. First, the spec sheet:

-14.5 1:9 Bushmaster, Brand new
- Q3131A ammo
- ARMS 40 A2 rear sight

I'm using the large aperture and I'm trying to zero so I can cowitness an Eotech. So! First string is high and left and a decent group. But every other shot is still high and left, regardless of what I do to the front sight post or windage. I cranked my windage almost all the way but it still shot in the same place.

This is the second time I've taken this thing out, and it's given me the same problem both times (We usually zero, then move on to pistol distances). Oddly enough, my friend's 14.5 bushmaster has pretty much the same problem.

Maybe I was just pissed and my shooting is shoddy, but my groups the second time out were bad as well. The mini-14 was getting much tighter! I shot it and a 20in colt (with a1 sights) and I'm dead on with those so I dont think it's my shooting (although thats debatable).

Don't want to turn this into bushmaster/ar bashing; it runs fine with magpul gi mags, especially for being brand new.

1) Maybe I'm having problems with the large aperture?
2) Canted front sight?
3) BUIS is crooked on the rail?
4) Bad batch of q3131a?

Help!

Try and zero the EoTech and then figure out which way you need to adjust things to make your irons line up with the center dot on the EoTech.  

Unless your rear sight is against one side or the other there is no reason for this other then the guy behind the trigger...  You can send me your upper with a few hundred rounds and I will zero it for ya

Quoted: . But every other shot is still high and left, regardless of what I do to the front sight post or windage. ... 1) Maybe I'm having problems with the large aperture? 2) Canted front sight? 3) BUIS is crooked on the rail? 4) Bad batch of q3131a? Help!

Canted front sight and crooked BUIS would produce consistant results.  You are seeing issue no matter what you do.

Sounds like you have an issue with the large aperture.  Try using the small aperture.  If that works then we can talk about getting an aperture that is larger than the small one, but no so big you have a problem with it.