Answer 1. Not really...read below.
Answer 2. Read below.
5.56 NATO vs .223
The 5.56×45mm NATO (official NATO nomenclature 5.56 NATO) is an intermediate cartridge developed in the United States and originally chambered in the M16 rifle.
The comparison between .223 and 5.56 isn’t a new one, so there is a lot of discussion to weed through to learn about the two. Ultimately, because they originated from the same cartridge, they are very similar, but that does not mean that they are necessarily interchangeable.
At a glance, the two rounds are indiscernible. Both rounds use a bullet of .224in in diameter and an overall length of 2.26in. In general, the external dimensions for the two calibers are identical. What’s more significant is the pressure of the two rounds and the difference in the rifle chambering.
One of the problems with comparing these two cartridges is that they utilize different methods of measuring pressure. SAAMI (Sporting Arms and Ammunition Manufacturers’ Institute) measures the .223 pressure at the center of the casing, whereas the NATO standard measures the pressure at the throat (or leade) of the chamber. To rectify the two different methods, several have undertaken experiments with their own standardized recording method to compare “apples to apples.”
Research confirms that, generally, shooting .223 through a 5.56 chamber results in lower pressure, but still functions (safely). Firing 5.56 through a .223 chamber, however, results in somewhat higher pressures. Although the differences aren’t massive (~5% in the previously referenced study), extensive firing of 5.56 through a .223 chamber could lead to over-pressure malfunctions, such as popped primers or blown cartridge case heads and other firearm malfunctions.
The most important difference between .223 and 5.56 chambers is the length of the throat (or leade) for each chamber. More specifically, the leade is located at the mouth of the barrel before the rifling occurs. Comparing the NATO and SAAMI regulations, the leade for 5.56 chambers is nearly twice as long as that of a .223 chamber (.162in to .085in, respectively). If a 5.56 round contacts the barrel rifling too early, it can cause pressure spikes (leading to malfunction, and potentially damage) in the chamber. This explains why it is safe to fire .223 through a 5.56 chamber, but not recommended to fire 5.56 through a .223 chamber.
As with so many elements of making a firearm purchase, the “right” option is subjective. If all you intend to purchase are .223 Remington rounds, there is nothing wrong with getting a rifle chambered for .223. However, if you want the option of firing milspec 5.56 through your rifle, you may prefer picking up a 5.56 chambered rifle. After all, you can still fire the .223 through it, safely, if you want. Price and personal preference are also factors to consider when making that decision. What works for you?
the .223 is slightly sorter of a round. SLIGHTLY. usually the 556 is a stronger round. if you have a .223 upper on your AR, some people will say that the 556 round will build up a little extra pressure due to the fact that the round actually touches the rifling before it is ignited. This creates a little extra friction or resistance when fired. There has never been a report of actual damaged caused by a 223 firing a 556 round. Overall they are the same.