Note: although never a soldier, I've played one on TV (and fired the M16 as well as a few other fun toys).

One reason for 5.56mm (not caliber) ammo is that it's lighter than, say, 7.62 mm or .303. A heavier bullet requires a heavier charge, which may require a heavier casing, etc. you can carry a bunch more 5.56mm ammo around with you. This is a Good Thing.

The primary advantage of the M4 is that it's smaller, with a carbine-type stock. I believe it's mostly being phased in (at least for now) to the light infantry units such as the 10th Mountain and 82nd Airborne. Even in those units, however, there's a few M16 carriers who have the M-203 grenade launchers.

Many of the well-known early problems the M16 had (it jammed, it blew up, yadda) were in fact caused by a combination of factors, not least of which was that Olin America (who, I believe, made the ammo) used a powder mix with a good amount of calcium in it. This tended to collect in the weapon, and when combined with the carbon present from combustion, formed...yup, limestone. This, coupled with the M16's less-than-handy cleaning charateristics (it comes apart real easy; getting the doohickeys into all the thingamajigs and scrubbing is the problem) meant that it would quite frequently jam up or even blow back. Cartridges would strike lumps of calceous deposits in the gun and refuse to seat, load or eject properly. Since then, different compositions of propellant as well as new cleaning guidelines (and a few design changes) have pretty well dealt with this issue. Roninspoon also reminds me that the design spec for the barrel was changed to include a chromed interior barrel lining to reduce build-up.

Finally, as to full-auto - why would you want it? You'd really prefer your riflemen to at least pretend to be taking aimed shots, since they have limited ammo. You have SMAWs and SAWs for suppressive fire. In any case, you'd just have to clean the weapon sooner for less useful time on range/in the field.

I only had the chance to play with the M16A1 during basic training, but but my impressions were favorable. It's light, accurate, and how can you not like a gun with parts made by Mattel! Yes, that plastic/fiberglass stock has the toy company's name stamped right on it.

Hermetic's complaint about an off-center grouping usually indicates that the sight-post needs to be adjusted. This is really an individual adjustment, my setting won't necessarily work for you and vice-versa.

Karmaflux is right on target: The M203 grenade launcher is the ultimate toy. The great thing about the M203 is that the 40mm rounds travel slow enough that you can easily watch them traveling towards their target. After a few minutes practice I could reliably put a shell into the window of an APV at 100 yards.

Of course no military rifle is complete without a bayonet. The M16A1 had one - the M7 (I think the A2's is an M9). Very dangerous little accessory to be handing out. From personal experience let me tell you they keep these things SHARP! I know the day we were first issued it there were lots of trainees standing around with bloody fingers - just couldn't resist the urge to test the edge and find out for themselves.

The Custodian mentions that the M-16 had some problems when it was first introduced, but there's more to it than that.

As designed by Colt, the M-16 was a highly reliable weapon. But getting it adopted by the army had much more to do with politics than with whether the gun actually worked or not.

One of the political problems was that Colt spec'd the M-16 to use a modern smokeless powder, and the boys in Army Procurement didn't like that. They'd always used something called ball powder, which was manufactured by a company just down the road, and they didn't see any reason why their buddies should lose the bid'ness on account of this new-fangled M-16 weapon.

So they took the M-16 and put it through its paces, looking for any excuse to disqualify it. And they found one. Seems that Mil Specs called for a muzzle velocity of 3000 ft/s across the entire temperature range of -30C to whatever, and if you took the rifle to Alaska and fired it at -30C ambient, the muzzle velocity was only 2950 ft/s, and there you go: it doesn't qualify.

BUT, they also discovered that if you used ball powder, the muzzle velocity meets spec at -30C. Problem solved.

Except for the other problem, which is that the weapon didn't work at all with ball powder. There were actually two problems. The first was the limestone build-up described above.

The second problem was the firing rate. The firing rate is a basic design parameter of an automatic weapon. As designed by Colt, the firing rate of the M-16 was around 800 rounds/minute.

Ball powder produces a higher chamber pressure than the powder that Colt recommended. Chamber pressure matters, because that's what drives the repeating mechanism of the gun. With ball powder, the M-16 fires at over 1000 rounds/minute; or, more to the point, it jams at over 1000 rounds/minute.

When the gun fires, there are parts that have to move: springs, levers, the bolt, the spent cartridge, the fresh cartridge; and at 1000 rounds/minute, these things just don't happen in time. So the weapon jams.

Colt tried to qualify the M-16 with ball powder, and found these problems, and told the Army in writing that the gun did not work with ball powder. The Army wrote back that they didn't think that the problems with the M-16 had anything to do with the powder, and that Colt could qualify the weapon with any powder they liked.

So Colt qualified the M-16 with the powder that it was designed for, the Army sent the M-16 to Vietnam with ball powder, the guns jammed, and soldiers died. Soldiers who didn't die wrote home to their parents, and parents wrote to congressmen, and eventually there were congressional hearings.

The hearings were a good illustration of the ultimate banality of evil. Here was a very bad situation: incompetence; malfeasance; corruption; lots of body bags. And when the Congress of the United States tried to find out what happened and why, all they got were these petty bureaucrats testifying that, yes, when fired with the manufacturer's recommended powder, the M-16 muzzle velocity at -30C is...

The Army never did back down on its commitment to ball powder. They reduced the calcium content in the powder to prevent limestone formation, and they changed the stiffness of a spring in the M-16 to bring the firing rate back down to 800 rounds/minute, and they went on from there.

It is also worthy to note that the M16 comes in 4 or more different variations (or more if you include the numerous carbines that use the same receiver and action).

The original is the M16. It was only in service in the US Army for about 4 years, from 1963 to 1967, as it was a not very clean weapon, and was falsely advertised to be self-cleaning, meaning that in the jungles of Nam, it was never cleaned(above writeups can explain this), thusly causing a terrible number of misfires and jams, because of Eugene Stoner's "ingenious" idea of feeding the gas directly into the receiver, rather than using any sort of lug system like the AK-47. It also only had a 20 round straight magazine like its M14 big brother. Another problem is that the gun is not ambidextrous; if you try and fire it left-handedly, the hot brass will hit you right in your eye. Not a good thing, especially for us lefties.

Then, in 1967, the M16A1. This version was really what made the gun useful in the field. They added a little button called a forward assist which should be pushed in after reloading, to lock the bolt into place incase it didn't get locked all the way after firing. This version added in the now-standard slight-curved 30 round magazine(as opposed to karmaflux's 40 rounds). In 1970, cleaning kits were rammed into the cavity in the butt of the gun and training programs were started to ensure that soldiers cleaned their rifles properly. The powder in the cartridges was improved. Things were looking up.

In the late 1970s, FN introduced to NATO a more powerful round, the 5.56x45mm. It was a better cartridge than the old .223 Remington of the old M16, so Colt developed an M16 to fire this new cartridge, the M16A1E1, with a heavier barrel, tighter rifle twist, a more advanced rear iron sight, interchangeable round handguards (instead of the non-interchangeable sticky plastic triangular one of the M16/M16A1), longer buttstock and furniture made of stronger plastic, and the now-familar 3-round burst mode instead of full-auto. In 1983 this weapon was adopted officially as the M16A2 5.56mm rifle, which we all know and love. The M16A2 is the basis for the M4 carbine, basically a shortened version of the same rifle. Since the mid-90's, FN Herstal has been contracted to manufacture almost all new M16A2s for the US Military. So if you join the Army today(or since 1994), you'll get an FN M16A2 rather than a Colt M16A2.

The M16A3 is just an M16A2 with a Picatinny-Weaver rail on top of the receiver, under the removable carrying handle, and a full-auto option instead of the 3-round burst. It is not in any US Army or Marine Corps inventories, but you Canadians may know it as the Diemaco C7. This is the basis for the Army's automatic carbine M4A1, which also has the rail systems on top of the receiver and automatic fire.

The M16A4 is the latest addition to the M16 family. It has rail systems on top of the receiver, like the M16A3, but also has them along the handguard, for mounting numerous accessories, such as optical sights, visible or non-visible(infrared) laser pointers, forward handles, grenade launchers, visible lights, etc. This gun can accept all of the SOPMOD gear of the M4A1, plus all of the Land Warrior or Soldier Enhancement Program gear such as cameras for peeking around corners and computers which connect to the user's helmet. The weapon is being put into use in the US Marine Corps in Iraq (Most of the pictures you see of fighting Marines in Iraq are using M16A4s - look for a tactical grip in the front, and a split up front handguard), much like the M4 and M4A1 are into the US Army.

It should be noted that the M-16 family of weapons all spring from the original AR-10 design. The AR-10 was chambered for 7.62 rounds, featured a heavier barrel, and lacked a forward assist (allegedly because it was not needed). The AR-10 looks like a slightly larger M-16. A civilian version is still produced with a semi-automatic fire rate.

Rumor has it that the Air Force was originally interested in the rifle but asked that it be rechambered for 5.56 (.223) rounds for training purposes.

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