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I went to the my local range today, and re-sighted my BM M4A2. I sighted mine in at 50 yards so that it would also zero in at 200 yards. I didn't have much time so I shot 30 rd groups after sighting in. At 75 yards it was almost 4" higher center of aim. Shooting prone at 100 yards it was almost 7" above center of aim. Which worked out great because I was able to place the target just above the front sight. Leaving me with a clear picture of the target. I have shot well over 3500 rds through my BM M4 A2. My groupings have steadily increased. Maybe I am just becoming a better rifleman, or maybe my rifle is.
Shooting bench at 50 yards my group of 30 rds was just under 4", and at 75 yrds it was just under 4-1/2", but shooting prone at 100 yards it was 6-1/2" tall and 4" wide. Please keep in mind that I was shooting with iron sights. I was once again very pleased with the results. BM's AR15s are by far one of the worlds best shooting weapons available for the price.
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Yeah that sounds alot like my experience FL.
You see my first two groups are about 4-6" high at 100 yards. This is after sighting in at 50 yards(IBZ).
However for a 200 meter IBZ zero one should only be printing a maximum of 2" inches above point of aim at 100 yards.
I had to drop my sights back to 1 click up elevation to print in the right spot(1.5-2" above P.O.A. at 100 yds).
Like Chuck says, that's why it's always important to verify ones 50 yard zero at longer ranges, preferably at 200 meters which is actually the correct zero.
Like I said you can do this roughly at 100 yds, just make sure your P.O.I. is 1.5-2" above P.O.A.
At 4" high at 100 yards, your zero is more like 300 meters instead of 200 meters, which isn't a problem as long as one remembers to place the target above the front sight post, and doesn't expect pin point accuracy out to 300 meters.
At 7" high at 100 yards, your zero is more like 375 meters, which equals almost 10" high at 220 meters. On a normal size dog that's the knees.
If you don't have a 200 meter range to verify your 50 yard zero and you want the real IBZ, go back to the 100 yard range and adjust your sights to print point of impact at 1.5-2" inches about point of aim. This will put you right in the bullseye at 200 meters(true IBZ)at point of aim and allow you to be within 2" of your point of aim all the way out to 220 meters(which is the whole reason for the IBZ).
Otherwise your range scale beyond 220 meters on your rear sight wont work.
The trouble with only using a 50 yard sight in zero without verifying the elevation sight setting at the zero range(200 meters), is it is easy to make a small point of impact elevation error at closer ranges that equals a big mistake at longer, or at ones actual zero range.
Confused yet?
That's why zeroing ones weapon at short ranges is really just to get one on the target at the actual zero range. Therefore, if you desire point of impact, point of aim accuracy, you must also zero the weapon at its actual zero range. For the IBZ this is 200 meters.