There is enough difference in performance between the 5.56 and .223 to warrant some consideration if looking at the two for a specific purpose. In some cases, you will want to be very picky about the type of round you are shooting based on your desired external and terminal ballistic performance. You may find that you need a certain type of projectile for hunting or defense, and it may come down to the performance of each. Now right off the bat, I want to inform you that the only reason these cartridges have different labels is because there are loading differences. We must also note that in some of the chambers made specifically for the .223, the beginning of the rifling is cut sharper, which can cause some pressure issues for the rifle when using 5.56 ammo. But we will get back into this later. On to where these rounds came from and how the two came to be the way we know them today.
The history of the two cartridges (5.56 and .223) really started when the military was looking for a small caliber round that could be used in their “Individual small caliber assault rifle” research and development project. They started with the .222 Remington cartridge and after failing some tests, Remington redesigned the round and named the new design the .222 Remington Special. This design ended up being renamed the .223 Remington. After passing the tests for the round satisfactorily, the military adopted the cartridge under the alternative measurement designation of 5.56 NATO and has remained the same since, for the most part. In the 60’s, Remington made the .223 Remington available to the commercial market where SAAMI then established standards for the cartridge, as with all other cartridges.
The 5.56 and .223 have no visible differences when you look at the two side by side, nor dimensionally in any way shape or form. The only dimensions that would be different, would be in the chamber of the rifle you are using, as I stated before. The case thickness is also the same, unless we are talking about steel/polymer hybrid cases, which are listed as a good alternative case manufacturing material to brass that wont cause problems despite range legend. The difference that typically makes the .223 lower in pressure is that it is regulated by SAAMI specifications, which generally lower the pressures of projectiles all over the market. If we look at NATO and European standards, their ammo is typically loaded to higher pressures. In some cases, they would cross into the realm of +P ratings if they were to be evaluated per SAAMI standards. For example, the 124gr 9mm NATO round typically is expected to go around 1200FPS out of the Beretta M9 Barrel (4.9″) with muzzle energy reaching up to 400FT/LB. This is in fact quite normal for the European 9mm standards. This was actually the Original and standard loading when the 9mm was first developed. Most European firearms chambered in 9mm are built to withstand these pressures as a normal diet since that is the standard pressures the 9mm is loaded to.
SAAMI specifications typically water down the energy and velocities of rounds, and the .223 Remington falls victim to this. The .223 Remington cartridge under SAAMI is loaded to about 7,000PSI lower that 5.56. This is the reason that you are advised against loading 5.56 ammo into a .223 chambered barrel. Typically the barrels for each caliber is designed to cater only that specific loading and using the 5.56 and .223 interchangeably in a .223 specific barrel can cause excessive wear and possible damage. But in most cases, a little bit of 5.56 wont hurt your rifle, just like how a little +P in your pistols aren’t going to hurt them. Based on your rifle, your needs for pressures and loadings in general may vary greatly. A 20″ barrel will have different flexibilities when compared to a 10.5″ barrel. Just keep that noted in the back of your head if you are weighing those 2 options.Personally, even though I have a 5.56 barrel, I like to shoot .223 ammo when I operate operationally on my range in my backyard, so I don’t think I’d really care if my rifle took 5.56 or not. It is inexpensive and is easy to shoot, so I will shoot .223 all day, any day. But my rifle (IWI Tavor) was recommended to have a 5000 round break in with 62GR 5.56 ammo due to the tight springs. That could be a big thing to consider if you have a new rifle and need to break it in. The higher pressures may just be what the doctor ordered when breaking in a rifle quickly and properly.
The biggest difference between what the 5.56 and .223 rounds offer is external and terminal performance based on velocity. Now if loaded the same with identical pressures and projectiles, the differences would be practically unnoticeable for the most part. But as the market stands, not everyone loads their own ammo and it is up to the manufacturers to load their ammo to whatever loadings they feel is safe but still practical. The .223 has a wider variety and flexibility when it comes to soft point and hollowpoint. There are a few companies that load soft point projectiles to 5.56 pressures with success, such as Speer. You may find that most 5.56 loadings consist of heavier weight bullets, while .223 being a varmint round primarily by design, will tend to cater to lighter loads.
There is one thing I have noticed that the 5.56 and .223 have in common. I found that it is hard to find good self defense ammo on the market that doesn’t have an issue in one corner or another. One round will be a shallow penetrator and not even reach the vital organs, such as the 5.56 and .223 TAP FPD and V-Max rounds by Hornady. The other issues you will find is that the ones that will get sufficient penetration will overpenetrate or just go off course and pitch and yaw on impact with flesh or bone like the M855 and M193. The temporary wounding cavity from a rifle round can have an effect on tissue and cause damage, but its useless if it doesn’t reach the vitals in the chest cavity. The point of using a firearm for defense isn’t to wound or hurt, it is to use deadly force because all other options have been exhausted. The only rounds that I know of that have sufficient penetration without much deviation and have good terminal performance overall is the 5.56 Speer Golddot soft point ammo. I prefer these rounds in the 64GR loading for my 16″ barrel rifles. If you use an SBR, I would opt for the 55GR variant.
The differences between the performances of the 5.56 and .223 are minimal but still are enough to make it a fine science to find out which one works best for you. For me, I think I prefer 5.56 when I can get a deal on it, but if not, .223 does just fine and I believe most of the AR15/5.56 rifle community would agree. Personally I like shooting hot ammo sometimes, and the 5.56 makes a great ammo for breaking in a new rifle, and as a defense loading. The .223 for me is a fun/budget practice round that can also serve as a varmint round of choice here in Alaska. Ultimately it is up to you to decide which loading you want to go with.