Yikes, this node CAN'T be empty...here's something until someone supersedes it!
Assault rifle is a term which, in the U.S. at least, is mired in the confusion of domestic political agendas. However, for the purposes of this writeup, let's think about what it means in comparison to other weapon types.
At base, the assault rifle is a shoulder-fired weapon intended for military operations. It is differentiated from the normal rifle by its name - how so in practice? If we examine the first use of the actual term, we find ourselves with the German Sturmgewehr 44, or StG-44. Sturm - assault, gewehr - rifle; there we have it. This weapon was differentiated from its contemporary shoulder arms (those in use by the Wermacht) by the fact that it was capable of fully automatic fire.
So it is differentiated from rifles by its ability to fire as an automatic weapon, from true machine guns by its ability to be fired from the shoulder, and from smaller weapons with this capability such as submachine guns by both its shoulder firing position and its use of heavier ammunition than is used in semiautomatic pistols.
In fact, if we look at the muzzle energy of various current cartridges in use, we can see that the muzzle energy of the assault rifle cartridge typically falls in between that of the pistol round and that of the full-sized rifle round. The latter is most concerned with power and accuracy at distance; the former with limiting recoil in order to allow hand firing.
Anthony G. Williams, the current co-editor of Jane's Ammunition Handbook, tells us that he uses the following definition for assault rifle: "A military rifle, capable of controlled, fully-automatic fire from the shoulder, with an effective range of at least 300 metres."1 He offers an excellent general history of the assault rifle, which I recommend to you if you're interested in the subject.
The Federation of American Scientists (FAS) defines assault rifles as follows:
Rifles capable of single shot or automatic fire using a short cartridge providing accurate fire and more controllable recoil force than a standard rifle cartridge. By reducing the cartridge case and propellant, the cartridges weigh less and soldiers can carry more. These shorter rifles were developed in response to the recognition that most fire-fights take place at ranges under 400 [meters]. The small size of the assault rifle and its ability to fire at up to 800 rounds per minute has led to it being adopted by various forces as a replacement for the submachine gun.2
So. We have, then, a rough definition of the assault rifle. Where did it come from?
As noted above, the first gun which explicitly used the term was the Sturmgewehr-44, a German weapon from World War II. However, there were others prior to this gun which not only laid the ground work, but can be considered assault rifles themselves. The notion of a shoulder-fired automatic weapon had been an attractive notion to many of the world's militaries, whatever their actual opinions on its possibility, ever since the introduction of the first machine guns. The U.S. Army fielded the Browning Automatic Rifle in 1918. More properly an 'automatic rifle,' since it utilized a full-size cartridge (the .30-06 Springfield) it nevertheless almost fulfills all our criteria. It is typically fired from a bipod mount, but can be fired from the shoulder, albeit with terrible accuracy.
By the close of World War II, the StG-44 and the smaller Schmeisser MP40 (despite the latter really being a submachine gun) had begun to inform military development. The Soviet Union's capture of a number of German automatic infantry weapons led directly to what is likely the most famous assault rifle ever, the AK-47. Although not an exact copy of the German weapons (notably, it uses a different loading mechanism) it bears strong outward resemblance and followed the capture and analysis of German designs. The Soviets had had a fully-qualifed assault rifle as far back as World War I - the Federov Avtomat, produced in 1916. It was one of the first designs to use a medium-powered cartridge to cope with the problems of recoil and mechanism fragility - namely, the 6.5x50mm Arisaka cartridge, captured in large numbers by Russia during the Russo-Japanese War. Only a few Avtomats were made, but its influence is clearly seen in the AK-47.
The AK-47 uses a full-caliber cartridge, the 7.62x39mm. It is medium-powered due to it being shorter in length and lower in charge than a full-sized rifle bullet. As noted above, this power restriction is common to assault rifles due to both the need for the shooter to control the weapon in automatic mode, as well as to the limits imposed on the power of the gun by the reloading mechanism which allows automatic fire.
The Western nations chose, eventually, a different approach. Rather than shorten and download a full-caliber round, NATO settled on reducing the caliber of the bullet while increasing its velocity. The result was the 5.56x45mm NATO round, originally intended for the American M16. This design accepted much higher chamber pressure in order to increase the muzzle velocity dramatically, required in order to retain accuracy and stopping power at ranges of two to five hundred meters for such a lightweight bullet. In addition, the smaller bullet meant a lower recoil, which would (it was hoped) allow soldiers to put more rounds on a target, more accurately. Studies had indicated that the primary determination of whether a hit was scored in combat was much more dependent on the time of exposure of a target, and how it moved, than on the marksmanship of the shooter.3
How well each approach solves the various problems of an assault rifle is the subject of an ongoing, almost religious debate among military scientists, soldiers, and gun enthusiasts the world over.
Various high technology answers to the question "what next?" have already been offered. In the final days of the Cold War, the most futuristic looking one was an attempt to take the NATO 'small bullet, high rate of fire' philosophy to the extreme. Heckler & Koch fielded the G11, a polymer gun which used caseless ammunition. Later, however, it became clear that more capabilities, rather than enhanced ones, would likely win the day. The notion of the modular weapon is still in vogue; a gun that can fire a variety of ammunition, be used in a variety of circumstances, and yet retain enough commonality to simplify logistics and manufacturing. Despite recent attempts (such as the U.S. Army OICW and the French PAPOP)4 failing to overcome problems of weight, complexity, reliability and cost, the search continues.
1 - http://www.quarry.nildram.co.uk/Assault.htm
2 - http://www.fas.org/man/dod-101/sys/land/bullets.htm
3 - Hitchman, Newmand and Forbush, Scott E.: "Operational Requirements for an Infantry Hand Weapon." Operations Research Office, Johns Hopkins University. Chevy Chase, MD: 1952. Available online at: http://stinet.dtic.mil/cgi-bin/GetTRDoc?AD=AD000346&Location=U2&doc=GetTRDoc.pdf
4 - http://www.dfait-maeci.gc.ca/arms/Trends/section14-en.asp