I attempted to zero my BM XM-15 (16"bbl) yesterday on a 50 yd range. I tried to follow the IBSZ procedure, but was unable to adjust the rear elevation as described in the IBSZ (1/16" allen wrench in the little hole, but couldn't get it to move 2 clicks CW). So, naturally I left the rear elevation a 6/3 and shot anyway...was initially all over the place (off the paper) but eventually got the front sight and windage to put consistent (though not very tight) groups on the paper at 50 yds. Windage is deflected WAY WAY left (almost all the way), but seems to be working for me. I'm not real smart on the ballistics here (arc of the bullet over various ranges and how the zero affects that)

Anyway, my question is this... What is going to happen when I start shooting at 100 & 200 yds? What do I need to dial into the rear elevation to get the same result as I am getting at 50 yds?

Thanks for any and all advice.

If you are zeroed at 50, with the elevation knob all the way down, using the 0-200 (large) aperture, regardless of whether the rear blade is over to one side, it is sighted in.  The rear blade being over to one side is another issue (canted front sight tower) and would be that way regardless of zero method you use.

OK, If you zeroed at 50, then your still hold right on the target at 100 m, make no changes, but now you will be hitting the target about 2" high.  Fine, this is the intentention.

Still, hold right on the bullseye, at 200 m you will be back on bullseye.  At 250 you will be about 4" low.

So, you use the large aperture and the elevation knob all the way down for all shooting from 0-250, and just hold directly on the target.  KISS!

For 300 m, you flip to the small aperture and dial the elevation knob to 3.  400 m, dial the knob to 4, etc.

Thanks. I was afraid it would be all out of whack if I tried to shoot farther than 50 m. So, if I understand correctly, use the large aperture less than 300m, small aperture 300+ m.

I forgot to mention, I used the small aperture when zeroing. Is that going to make a difference?

What can I do to correct the canted front sight tower, or is it even worth worrying about? Once dialed in, it's shooting as straight as my skills allow.

Well I recently took my RRA 20" out to the range and did a 50 meter zero, so I might be able to give you some insight.  For mine I did not do any of that fancy adjusting of the rear windage, I just set my rifle on "Z".  I did my zeroing at 50 meters and after about 12 shots got it zeroed. Ok now on to the ballistics.  From what I know if you zero at 50 meters (roughly 50 yards) the path of the bullet at 100 meters barely rises an inch or less.  Then at 200 meters it comes back to zero, so at 200 meters you can aim center mass.  Then at 300 meters it is about 7 to 9 inches down.  This is all with out adjusting the rear elevation; I leave it on "Z".

I was hitting pop can size targets with irons from 50, 100 and 200 meters by aiming center mass at all of them with a 50 meter zero.

The reason the 50 meter zero is so nice is because it allows you to aim center mass of a target from 0 to 200 meters which for me getting ready to hunt some coyote is what I am looking for.  It is much nicer than the army’s 300 meter zero, because it does like I said allow you to aim center mass from 0 to 200 meters, as where with a 300 meter you can only aim center mass at 300 meters then a 400 and 500 with the proper elevation adjustments.

But honestly 0 to 200 meters is what I find to be the most effective range for any tactical or hunting situation, because past 200 meters the 5.56mm tends to lose its “explosive” capability.

Note: I use the small apeture all the time.  I zero with it, beucase hey that what I was tought in the army and it has always worked for me there.

If you are sure the windage is off and its not shooter error,send your upper back to Bushmaster they will fix it for free.Maybe if they get enough of these back they will get there heads out of there a&&@$ and start getting there sight towers on staight.They say they boresight them but I do not believe it! The rear sight on my Bushy required 21 clicks of right windage to zero when new .I called customer service and the rep fed me some bull$#!% about the index pin being sheared off,I sent it in and when I got it back it only required 7 clicks of right windage to zero.What they did was pound the front sight taper pins out center the sight redrill the taper pin holes a little deeper then drive the taper pins back in.It would be a easy fix with the right drillbit.

Awesome. Thanks for the great info, guys. At first I was immensely frustrated with my BM because I couldn't get it zeroed (novice here), but I think I finally achieved success. Now I can't wait to get out and try it again! Cheers.