Today, I was zeroing the BUIS on my Carbine and noticed that my rear aperture changes zero depending on whether I’m using the large aperture or the smaller aperture.
My shots are always high when using the small aperture.
I’m using the MI ERS rear BUIS and their front rail mounted BUIS as well (I’m using a 13.2" rail, if sight radius is important).
I’ve already had to saw a section of threads off, saw off a section of the detent, and clip a few coils of the detent spring so that my sights could be adjusted low enough to zero.
Now, it seems the large and small apertures are sighted for different ranges. At 50 yards, the small aperture shot about 3" high.
What’s the issue here, and if they are sighted for different ranges, what range is my small aperture now zeroed for?
That’s why some BUIS’s have what they call “same plane” apertures. You pay a premium, but you avoid the shift.
MOA shift is dependent upon sighting distance length (the shorter the distance between the rear and front sight, the bigger the shift).
Establsih your 50 yd zero using the small aperture. Then flip to the large for close range engagements or when lighting conditions are poor. Make note of the MOA shift between the two apertures and adjust holdovers accordingly.
I zeroed with the large aperture because it is the first one up when the BUIS is flipped, to me the whole point of a BUIS is for those “oh crap!” moments where the optic fails and the gun is still need RFN. I may change that, but it makes sense to me, for now at least. I understand that I may be better served by using a zero with the small aperture for precision reasons, but I have my BUIS for those moments where flipping the sights up may take more time than I have, let alone flipping up the small aperture. Any more info is appreciated, I wanna get to the bottom of this, its been bothering me for a long time.
Most large apertures will say 0-2 bellow them, as in 0 to 200 meters. The small aperture unless I’m mistaken is 300-400 meters, unless it is a same plane. Same plane means regulated to the same POI as the large.
You sight in with the large appeture and use it for 0-200 yards and flip up the small appeture if you plan on shooting 3-400 yards. That’s my understanding anyway, unfortunately my groups open up with the large appeture at 100 yards so I prefer the same plane appeture sights such as the Troy for a BUIS.
On my A2 carbine I have a Trijicon front sight post. I think I can zero with the small appeture at 50 yards, allowing me to use the large appeture at night centering the dot which is lower than the top of the post and should be close to being on. I just haven’t tried it yet to test out my theory.
“So if I zeroed using the 50-200 yard method, what range is my small aperture sighted in for?”
I think the “IBZ” is sighted using the small appeture.
The A2 and A4 rear sights will most times need to be adjusted using a 1/16" allen wrench so that you can get the elevation drum to bottom out at four clicks below 6/3 for an A4, or two clicks below 8/3 for an A2 rear.
The 50yd zero is using the small aperture at either 6/3 -4 or 8/3 -2 use the front for elevation adjustments and the rear sight windage for windage changes. If your rear sight doesn’t have elevation built into it just use the small aperture (Troy/LaRue etc).
Sight in for 50yd POA/POI.
Once sighted in then you’ll just leave everything there. You can switch to the large aperture for low light or very close shooting etc.
Gotcha. I’m not familiar with that model. With some sights you can reverse the aperatures around. I too prefer a flip up to come up with the small aperture.
I guess you could just leave it up and on the small one.
This really shouldn’t be a problem. If you are worried about a self defense situation, I’d be more concerned that in an “optics out” situation, that my setup requires me to manually deploy my front sight as well. That’s why I’m a fan of std, fixed FSB’s.
If the target is real close you should be going to your blaster (if you have one). Otherwise go with your BUIS. If it defaults to the large aperture, that’s OK because if your target is close, and quickness is of the essence, that’s the one you’ll want to use anyway (you won’t want to spend any time monkeying with flipping to the smaller aperture in that situation). If target is far enough away then you have the luxury of time, just make sure you’d be justified using deadly force against a far away target.
I’d really suggest you try operating your carbine with front sight up and verify whether or not it screws up your sight picture when using your optic. Then just practice deploying the rear BUIS and seeing if it’s that big of a problem. I’m thinking in the end it won’t be as big a deal as you think it might be, but the only way for you to ascertian this is by practicing.
I use an Eotech mounted to the rail for absolute co-witness. I ran it for about a year before I got my rail system with the flip up sight. I think the front sight gets it the way a good bit, but it’s not impossible to run it in the up position. I could run my whole setup with the irons up if I had to, its just not that big of a concern.
I think it boils down to this:
Do I want my large aperture to be on target, so that I can flip my BUIS and be ready to go?
Or do I want my small aperture on target so I can shoot more precisely, but at the cost of my BUIS being off target when its flipped up initially?
I’m leaning towards the large aperture since the odds that Eotech can crap out and I’ll need a quick shot with the large aperture within 50 yards are pretty rare, but the odds of my Eotech crapping out and me having the time to get my small aperture up is rarer.
OTOH, I could settle for inaccuracy with the large aperture sine it’s for speed anyways and zero the small aperture since it’s the only one worth a damn in the accuracy department.
I’m just trying to think of when I need my BUIS and the first thing that comes to my mind is that I need speed on my side. That’s what I like about the MI sights. They deploy very smooth, very fast. They aren’t spring loaded, but they deploy very fast. I can get both sights up in around a second.
When you flip up the sight flip the aperture. If the BG is that close that flipping the aperture is hindering you, you should be firing already anyhow. I use a MI rear and after lifting the sight I use my thumb to change aperatures on the way back to the pistol grip. Besides if you are using a quality optic the odds on you using the BUIS are very small.
If you have a friend who’s running an EoTech mounted on a riser, I recommend you see if they can let you try that combo out.
Like you, I ran an EoTech mounted directly to flat top which provided absolute co-witness. I also ran a LaRue BUIS, I thought that made the most sense and I didn’t think the sight picture with the irons and the EoTech was all that difficult to deal with.
Then I went to a local going out of business sale and noticed the owner had a YHM EoTech riser marked down. On a whim I bought it. All I can say is “WOW”, I am now a convert of lower third co-witness. Even with the fixed sights, the EoTech’s reticule is above all of the irons, it’s really a better setup IMHO. I highly encourage you to seek out someone who’s running a similar set up and try it out. Assuming you like it, then just run your front sight up and deployed all the time.
If you don’t require quick detach capability, then a YHM will get the job done just fine. If you require QD, then go with a LaRue.
If you have a full size rifle and want to use your battle range, then the range drum doesn’t need to be adjusted. Your gun should come with it bottoming out at 6/3 or 8/3 from the factory.
If you are using a carbine, then an “overall” zero like gotm4 described is better as you wont be using the battle range adjustment on your carbine.
the big hole on an A2 should lower the shot. missing high the bad guy walks away. missing a couple of inches low your still on target. it is better to hit than miss. they made it to work that way. get a same plane aperature to fix this.
AR-15/M-16: Rear Sight Elevation Shifts and the need for a Same Plane Aperture Sight
found this–As originally designed the AR-15/M-16 sight system adjusts for windage at the rear assembly and for elevation in the front to zero the rifle to the individual shooter. To obtain the ballistic adjustment needed for longer range shooting the original rear sight had one aperture higher than the other. While this system would not give the shooter an exact elevation adjustment for a specific range, it would under battle conditions put the bullet close enough for government work so to speak.
The new A2 rifles and carbines employ a rear sight system that includes a range cam to allow the shooter to more precisely adjust the ballistic arc to their needs. This system eliminated the need for an elevation shift between the large close quarters aperture and the smaller long-range aperture. However the aperture shift is still with us today.
Comparing some of the rear sights available today there appears to be 3 variations available. The 1st style (and oldest) has a .014 offset. This gives a 2.52 inch shift in point of impact at 100 yards between the two apertures. The 2nd style has a .017 offset, which shifts point of impact 3.06 inches at 100 yards. The 3rd style has a .024 offset giving a 4.36 inch shift in P.O.I.
All of the points of impact shifts shown above are for the standard barrel AR-15/M-16 that has a sight radius of 20 inches. The shorter sight radius (14.5 inches) of the M4 carbine and weapons of similar configuration increase the amount of point of impact shift. The shifts for these shorter sight radius weapons are 3.48, 4.25, 5.96 inches respectively.
The only reason for the various shifts would be for changes in the ammunition used by the military over the course of this weapons employment.
Most people are completely unaware of this elevation change that occurs when the aperture is shifted from one to the other. They just figure that their rifle is doing something strange.
Our aperture sight for this weapon has both the large and small apertures on the same plane so there is no difference in the point of impact when you shift from one to the other. We have also offset one of the apertures by .007 to make up for the sideways movement of the aperture on the windage screw as it pivots.
By installing our same plane rear aperture you now can sight in your rifle using the smaller aperture and shift back and forth between the two apertures as your light and range needs change without having to do mental gymnastics to know where your bullets impact will be.
