I'm a newbie to AR15's and this AR15.com so please take it easy on me.  I have scoured these pages for days trying to soak up as much info as possible and everyone seems to be very helpful.  I bought a new (stock) Bushmaster (16") with A2 sights.  Before buying buying a scope or laser device I wanted to make sure I was profficient with the iron sights.  So, I follwed BM procedure of zeroing in at 25 yards using the small aperture (the rifle was shooting very high, had to click the front post down about 5 times to get it right).  I thought everything was cool, getting about 1.5 to 2 in. groups at this distance.  I was shooting at an indoor range and 25 yards was max.  Today I went and shot about 100 rounds at a target I paced off to be about 85 yards.  The first 30 rounds I used the small aperture and had marginally acceptable results.  Then, since the manual says to use the larger aperture out to 200 meters i decided to give it a try.  All of a sudden my shot group was about 8 inches high and most never even found the target.  Although this AR15 thing is new to me, I have shot rifles  darn near my whole life, I might not be an excellent shot but I am not bad either, or least this bad.  Can anyone help me with this?  What am I doing wrong.  Been using decent ammo, BH 55 FMJ or XM193 55 FMJ.

correct me if I am wrong, but if you are shooting high, you need to raise the front post, not lower it.
is this a flat-top rifle or a non-removable carry handle?

Sorry, it is the A3 removeable handle.  I'm pretty sure that I rotated the front post correctly because when first zeroing I was able to bring it down by rotating counter-clockwise.

Also, when zeroing at 25 yards I follwed directions by setting the elevation knob to 300-meter mark (6/3 in this case) plus one "click" up.  Then I would put the 300-meter mark back to zero or standard for the 85+ yard shots.

Yes counter clockwise is raising the sight post.  clockwise is lowering it.  Just like a normal screw or bolt.
flat tops are known for shooting high, see this thread:
www.ar15.com/forums/topic.html?b=3&f=18&t=247832

I actually called bushmast this past monday to get a .040" taller front sight and I just got it 10 minutes ago.  didn't have time to play with it.  when I called them they wanted me to send the rifle back and they said that it may shoot high with arms buis installed, I told them I had a troy installed but it was still shooting high with the CH as well.

Oh, I never messed with the 6/3 thing, plus I pulled my CH off anyway.

Thanks Jackal...BACK TO THE RANGE I GO...BUMMER!

BiggSlic,

I had the exact same trouble last weekend.  Sighted in at 25 yards, and was 8" high with the large aperture at 100 yards.  Someone directed me to the Maryland AR15 Shooters Site for zeroing techniques.  I'm going to give the 50-yard improved battlesight zero a try.  

There was also a post here showing the bullet rise for different zeros, I'll se if I can find it and post it here also.

Good luck!

ETA: Here is the post which shows the trajectory charts.  You'll see it's pretty close to what you are getting for the 25-yard zero.  

Not MrWizard, Thank you!  The Maryland AR15 site is very helpful.  When I went back to the range I was still experiencing the exact same problem.  Your link helped me figure out that the A3 site I am using is actually using "half clicks"...thus I was using the wrong point for zero on the rear site.  That 50 yard battlesight zero looks like it is for me too...I can't really see myself doing too many 400 or 500 meter shots.  THANKS AGAIN!

Quoted: BiggSlic, I had the exact same trouble last weekend.  Sighted in at 25 yards, and was 8" high with the large aperture at 100 yards.  Someone directed me to the Maryland AR15 Shooters Site for zeroing techniques.  I'm going to give the 50-yard improved battlesight zero a try.   There was also a post here showing the bullet rise for different zeros, I'll se if I can find it and post it here also. Good luck! ETA: Here is the post which shows the trajectory charts.  You'll see it's pretty close to what you are getting for the 25-yard zero.

Those charts (and tables) can all be found at The Maryland AR-15 Shooters Site - I have links from the Zeroing page to the charts.

Quoted: Not MrWizard, Thank you!  The Maryland AR15 site is very helpful.  When I went back to the range I was still experiencing the exact same problem.  Your link helped me figure out that the A3 site I am using is actually using "half clicks"...thus I was using the wrong point for zero on the rear site.  That 50 yard battlesight zero looks like it is for me too...I can't really see myself doing too many 400 or 500 meter shots.  THANKS AGAIN!

The nice thing about the IBSZ (50y) that is it will calibrate your sight to the other ranges properly.  Switch to '4' and you have a 400m zero.


Oh as to your other point about the zero shifing when you switch apertures - well it's supposed to.  The sight was designed so if your small aperture is set at 300M then flipping to the large aperture gives you a 200M zero.  There is also a problem that there seems to be at least 3 different apertures (having different elevation changes when the aperture is flipped).  You can solve that problem by going to XSSights.com and purchasing one of their 'Same Plane' rear apertures.  This unit will shoot to the same point of aim no matter which aperture you select (unlike the GI & commercial units).

Forest,

I understood that there is no difference between the large and small aperatures in terms of height for the A2 sight. Thus, there is no difference between point of impact between the two sights when the small aperature is used to acheive a 300m zero. The large aperature is useful for CQB and low light conditions (as explained by my USMC friends). Am I missing something here because it IS confusing having the larger aperature stamped "0-200" ? Incidently,  I do recall contacting BM tech support and learned that the large aperature should not change the zero of the rifle and that the large aperature is intended for rapid front sight aquisition.

Quoted: Forest, I understood that there is no difference between the large and small aperatures in terms of height for the A2 sight. Thus, there is no difference between point of impact between the two sights when the small aperature is used to acheive a 300m zero.

That is incorrect.  There is a difference and it was designed that way.  The biggest problem is the difference doesn't seem to be consistant (there seem to be at least 3 different offsets available).

That is why the XS Same Plane rear aperture is available.

Am I missing something here because it IS confusing having the larger aperature stamped "0-200" ? Incidently,

 
Do a simple test.  Zero the rifle at 50y (use a very large sheet of paper), then switch apertures and fire.  You WILL notice a change in the point of impact.  Another option is to break out the calipers and micrometer and do the measurements yourself - you'll see the center point for the apertures are at different heights.

I do recall contacting BM tech support and learned that the large aperature should not change the zero of the rifle and that

Well whoever told you that is wrong, and I have several BM A2 apertures that will prove them wrong.

BTW I should point out that BM Tech support has used the Maryland AR-15 Shooters Site as a reference to their customers on these kinds of issues.

the large aperature is intended for rapid front sight aquisition.

That is correct, rapid fron sight aquisition for close range and low light conidtions.

Watching, because I've had some confusion about sighting in. I guess I should have posted my question in this forum. Here's the link: my thread

I give a quick summary there of the A1 style apertures according to TM 9-1005-249-10. They came in two versions: in one version, changing aperture changed point of aim, in the other it didn't.

GL

Forest,

Thanks for all the replies on this.  A couple more questions if you don't mind!  Do you have any charts that will show what the expected POI shift will be after zeroing with the IBZ and small aperature, and then switching to the larger one?  Or is this just going to be a matter of collecting my own data?  

Thanks!

PS: And Bushmaster references people to your Maryland website for help??? What is wrong with THAT picture!!??

Quoted: Do you have any charts that will show what the expected POI shift will be after zeroing with the IBZ and small aperature, and then switching to the larger one?  Or is this just going to be a matter of collecting my own data?

You'll have to collect your own data as I don't know what the shift will be with your aperture.

One tip.  Follow the IBSZ with the small aperture at 50y.  Then flip to the large one and see where your rounds will land (they should land low).  Adjust elevation on the REAR sight till your rounds hit COM using the rear aperture.  Then check the setting (it should be around 3, but may be as high as 4 depending on your aperture).  That is the setting you should use when you use the large aperture.

Otherwise the small aperture will shoot to 200M when bottomed out, and to 300,400,500m when you select 3,4,5.

PS: And Bushmaster references people to your Maryland website for help??? What is wrong with THAT picture!!??

Nothing.  Bushmaster BUILDS the ARs, that doesn't mean they got all the scoop on various issues - nobody has all the information on all issues.  I take it as a compliment.

Quoted: Watching, because I've had some confusion about sighting in. I guess I should have posted my question in this forum. Here's the link: my thread I give a quick summary there of the A1 style apertures according to TM 9-1005-249-10. They came in two versions: in one version, changing aperture changed point of aim, in the other it didn't. GL

GL,
The A1 came in only one style the standard 250M/375M (L) system.  The Promethius sights (the other aperture mentioned) was a short lived experiment I never saw one when I was in the service, nor have I talked to anyone who used them.

However the XS Same Plane will give you no shift in impact when you flip the apertures if that is what you are looking for.  The Troy is the same way (no shift when switching apertures)

I'm unsure about the LaRues, but I suspect they use standard A2 apertures.  In that case if you don't want to change to XS, then zero at 50y with whichever aperture you plan on using (I'd suggest the large aperture - as the small aperture will hit to a higher point of impact).

Thanks Forest. I like the idea of just flipping from one aperture to the other and having two different zeros, like the old A1. So I'll pass on the Troy for now and continue looking at the LaRue and LMT. My upper is on order from Talon Arms, located in Houston, and as I type, Rita has not yet landed.

GL

I noticed that the Maryland AR 15 site that lists the IBZ instructions notes that sometimes when switching to the large apeture requires +2 or even +4 on the site wheel...

"10. If you're patrolling, set the sight to 8/3 and snap the aperture forward to 0-2. This will provide the same trajectory as above but with a larger, easier to see thru rear sight. Use this setting if you also have the M68 mounted as it's quicker to transition to if the sight fails. [Editor's Note - there is some variance with the offset of the A2 aperture - they SHOULD be a 2 click difference - however some manucatures produce them with larger offsets.  Setting the sight to 3 then flipping to 0-2 might now work for your AR.  Check it at the range, you want the group to be centered  at 50y, you might need to set the sight at 3 +2 or even 4 to get the large aperture to be correct]"

I think I would rather opt for the XS Same Plane site recommended by Forest.  Thank you Forest for your input.

Quoted: The nice thing about the IBSZ (50y) that is it will calibrate your sight to the other ranges properly.  Switch to '4' and you have a 400m zero. Oh as to your other point about the zero shifing when you switch apertures - well it's supposed to.  The sight was designed so if your small aperture is set at 300M then flipping to the large aperture gives you a 200M zero.  There is also a problem that there seems to be at least 3 different apertures (having different elevation changes when the aperture is flipped).  You can solve that problem by going to XSSights.com and purchasing one of their 'Same Plane' rear apertures.  This unit will shoot to the same point of aim no matter which aperture you select (unlike the GI & commercial units).

I didn't know this about the different heigth aperatures, I will have to start over at sighting in my Bushmaster A3.

How many turns up can the front sight move before it unthreads?

P.S. I apologize for the hijack

Forest,

Went out to the "range" today with my 10 month old BM 20" A3 shooting 55gr fmj. I confirmed my zero at 300m with the small aperture. Set the target at 50m and found that the large aperture produced groups that were 4" LOWER than the small aperture. Thanks for the help Forest.

Quoted: How many turns up can the front sight move before it unthreads?

Honestly it depends on where you started from.  If the base of the front sight pin was about level with the FSB then you should have plenty of room to adjust the front sight.