Here some tips about M16 and AR-15 rifles. These have been proven by the USAR Shooting Team and written about by SFC Hubert Townsend.
Most issue, rack grade M16/M4s are two or three minute of angle (MOA) guns. That means that at 100 yards they shoot a group covering a circle of two or three inches diameter, four to six inches at 200 yards, etc. That is normal but can't always be assumed. I was once issued a rifle that turned out to be only five MOA and didn’t discover it until shooting a five inch group at 100 yards on a paper target. No wonder I had trouble at the 300 yard line with it!
Confirm your weapon’s ability with a good, solid, prone supported shooting session. If it is excessively large get a known good shooter to confirm that the large group wasn’t your own marksmanship failure.
The US Army Reserve team discovered through testing that 10% of rifle magazines may cause a rifle to shoot to a different point of impact! I know this is a fact because we put my rifle into our machine rest and shot 10 round groups with different mags.
The first magazine shot the usual two inch group. We inserted mag #2 and there were now 20 holes inside the same two inch circle 100 yards away. Then we inserted mag #3. Good Golly Miss Molly - the entire group shifted two inches to the right! Wow, that explained why my shots at the 300 yard line were off to the side. It wasn’t me being stupid or missing the wind or a loose position. It was my magazine the whole time. So now, when I get a new mag, you can bet that I shoot it at 100 yards and ensure the rounds are hitting the same spot as the others. Also, you know why all my mags are labeled.
Lastly, for iron sight shooters, there is a very simple way to increase precise aiming. Standard Army rifles are set so that the rear elevation knob can't go below the 300 meter (8/3) setting. That knob can be adjusted with an Allen wrench to allow the rear sight to move below 300 meters/yards three or four minutes. This means that shooting at a 200 yard target, just click down two minutes, hold center, and that is where the group will go. If moving in to 100 yds, then click down one more and you will be dead on. No more worrying about holding high or low at different distances, just click the rear elevation knob. Much more precise. This is the Marine standard setting their M16s.