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.
Army Reserve Marksman 
The Corps has a great ability to train people to shoot and shoot well, both out of basic and while continuting to serve.
The lack of marksmanship training and an overall lack of verbal/financial support, time available, etc… in the Army Reserves was pretty shocking to me but now I generally accept that most units will have about 25% or so who just will not qualify or will do so only with the “help” of a pencil.
Everything I learned about shooting came from my experience in the Corps.
Most reserve units just seem to go thru the motions.
Sad.
Good blog by the way.
About 27 years ago, egads, can it be that long ago , I learned to test M14 magazines to make sure point of impact didn’t change with mag changes. Then, for competition, I had only TWO magazines; always the same #1 and #2. ((I still use them! Also, #2 was also drilled to show brass at 8 rounds* … “belts-and-suspenders.”))
Seems it’s true for M16s too.
There are two points here – for you grunts who have to qualify and want to earn marksmanship badges, it pays to “preflight” you equipment.
There’s an old saying, “It’s a poor craftsman who blames his tools.” Yes, that’s true. But it assumes the tools are properly sharpened before use.
I once worked with a young police sniper who, immediately after returning from an update class, couldn’t hit the broadside of a barn. He didn’t question his equipment, and the result was he lost his confidence and wasn’t much good to anyone. His SGT sent the man to me. I shot his rifle (a nice Remington .308 bolt gun). Guess what: I couldn’t hit anything either!! So I told him to let me borrow his rifle for a few days. On my range, it shot great: same hole at 100 yds. Hmmm. Same rifle, same shooter, different range? NO! I didn’t have his ammo!! Turned out the batch of ammo he had was trash for his gun. When he switched to what I was using – ZING – he was A-OK again. It was his equipment.
When at FLETC in 2000, same sort of thing happened. A federal LEO who was at firearms instructor school couldn’t shoot. Now his command wouldn’t have sent him there if he couldn’t shoot … but he couldn’t. Turned out his piece-of-trash pistol had a loose rear sight. The sight was wobbling left and right – so his shots were spraying left and right. Once that sight was tightened – ZING – he was A-OK again. It was his equipment.
At the link, you’ll find these words, “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.” Yes, it’s nice to get a 2nd, confirming opinion. But if you know how to do it right and YOU ARE DOING IT RIGHT, check you equipment (gun & ammo). It’s not always the shooter.
* The hole drilled in the magazine reduces the odds of the ‘rookie mistake’ of failure to put in 8-rounds. Just load ’til you see brass. When the magazine is inserted, the hole doesn’t show – so it’s a legal mod. Count AND ‘load to brass’ = “belts & suspenders”