IBZ stands for Improved Battle-sight Zero. The common drum elevation adjustable rear sight can be tuned to achieve some great versatility. The method for adjusting for A2 and A3 drums is a simple process. Before we begin remember that a detachable carry handle on a 20 inch barrel translates to a 1/2 MOA adjustment for windage and elevation. This makes a flat top 20 inch rifle with a rear drum a *very* precise machine. Contrast that to a fixed carry handle which adjusts against 1 MOA clicks for both windage and elevation. This doesn’t mean that A2 drums aren’t precise… but if you are going to give high power shooting or XTC a try with your fighting rifle *as is* then you would do best to take an A4 with detachable carry handle.
What the IBZ does is allow for adjustments to be made below the 6/3 setting of the elevation drum. Instead of using the large aperture for 0-200 meters, you can configure your rear sight with the IBZ method and get a working 50 meter zero using the small aperture and yet retain the adjustments of 300-600 meters. The 50 meter zero is amazingly useful. At the apex of a 62 grain bullet’s flight path with a 50 meter zero you will see about 2 inches of bullet rise. The projectile will cross your line of sight again at 215-225 meters roughly. It will then drop from 4-6 inches right before it crosses the 300 meter mark.
Simply re-adjusting to the 300 meter mark on the drum will alleviate that problem quickly.
Here is a quick tutorial for the IBZ:
Click on the first image to start the slideshow.
To adjust a A2 upper for the IBZ set to – 2 clicks instead of the -4 clicks above.
While the Revized IBZ is also popular at the moment I find that the IBZ is simple enough and that tuning the drum even more to get a 50 and 100 yard zero seems unnecessary. For targets at 100 meters I simply use a 6 o’clock hold with a -4 click setting. Furthermore adjusting a carry handle (or A2 drum) past -4 clicks can put us into potential error territory. If I want a 50 meter zero I simply use IBZ and bottom out my drum. It wont go past -4 clicks so I can’t go past or screw up my very useful 50 meter zero. If I were to use RIBZ i could adjust to -6 clicks below my 6/3 mark on my detachable carry handle. All that just for a 100 meter zero. No thanks!
The IBZ is simple, relevant, and the 50 meter zero is a great zero for the 5.56 cartridge. I hope you don’t need much more convincing if you haven’t done this already. 🙂
A2 windage is 1/2 MOA, not full MOA.
I did a version of this with my 20″ A2 rifle. Added 3 clicks instead of 4 since the elevation drum on my A2 is one MOA per click. So needed two clicks below 8/3 to get to 200 (50/200). The third click allows me to also accommodate a 100 yard zero.