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223 Remington vs 5.56mm NATO, and touching on 22 Long Rifle
223 Remington, 5.56mm NATO, and 22LR are somewhat inter-related calibers. 22LR can also travel down 223 and 5.56mm barrels, but the converse is not true. This is because 22LR barrels are actually .223 diameter, as opposed to.224 diameter.
For additional information on 22 Long Rifle, see RefARcal22lr
Market Considerations on Rifles
If you're trying to decide whether to buy a 223 Remington or a 5.56mm NATO chambered rifle, it definitely comes down to your personal needs. For the overwhelming majority of people and situations, the correct answer is 5.56mm. The only reason to buy a rifle chambered in 223 Remington over 5.56mm, is if you happen to be a shooter competing in the accuracy area, looking for the last slight edge in accuracy. The tighter chamber will marginally improve your ballistics at range over that of the 5.56mm chamber. If your competition leads more towards practical or tactical shooting, it's likely that you should choose 5.56mm for the slightly improved ease of extraction. The vast majority of AR-15 uppers on the new market are chambered in 5.56mm now.
It's worth knowing that the 5.56mm chamber's throat, or 'leade', is approximately twice as long as the 223 chamber's corresponding dimension. The throat is actually the conical, non-rifled part of the chamber which leads to the rifling. Precision rifle shooters will frequently measure the internal dimensions of their chamber and seat their hand-loaded bullets far enough forward to contact the rifling upon being chambered. This has a positive effect on accuracy, but must be done very carefully to prevent a catastrophic situation where the bullet gets 'stalled' on the grooves and gets held up on its way out of the case, thus allowing pressures to spike to a dangerous level before the bullet is pushed down the barrel.
Although they're almost impossible to distinguish in a pile of mixed ammunition, 223 Remington and 5.56mm NATO are different. Just different enough to blow up a gun chambered for 223 Remington, actually.
Historically, 223 Remington and 222 Remington Magnum were two rival cartridges for military certification. Functionally, they're more or less the same (although not dimensionally comparable). It's worth noting that firing 223 out of a 5.56mm chamber will actually be very slightly less accurate, due to chamber considerations. It's also worth noting that the actual barrel diameter is .224", even though the round is named 223.
Standardized one year before 223 Remington, the 5.56mm NATO round is functionally the same round with a higher rated pressure (up to 20k PSI higher!) and a thicker-walled casing (rated for higher pressure, but with less powder capacity). This is why it is unsafe to fire 5.56mm ammunition through a rifle which is marked for 223, unless the manufacturer states that the chamber is rated for 5.56mm. The Robinson Armory M96 is one example of a non-5.56mm-safe chamber. The 5.56mm brass is also thicker than 223. Like 223 Remington, the barrel diameter of a 5.56mm round is .224".
A Wylde Alternative
It's also worth noting that there's another chamber specification out there: 223 Wylde. This is a longer-chambered variation between 223 Remington and 5.56mm NATO. Rifles with Wylde chambers, unlike 223 Remington chambers, are safely able to fire 5.56mm NATO ammunition.
22 Long Rifle
The ubiquitous and inexpensive 22LR round can be fired through a large number of different rifles. It's also possible to design a conversion kit into any 223/5.56mm rifle, to shoot 22LR out of it. This is quite commonly done on the AR-15 platform with the 'Ceiner' conversion. One thing to be aware of, when considering a drop-in conversion, is that 22LR bullets are .223" in diameter, while 223/5.56mm bullets are .224" in diameter. While the 22LR bullet will readily travel down a 223/5.56mm barrel, it won't be a very tight fit and there will be a drop in accuracy compared to a barrel designed for 22LR. Another consideration is that the optimal twist rate for 22LR barrels is slower than any of the common 223/5.56mm twist rates.
-- SeanNewton - 07 Dec 2007