.223 vs. 5.56 mm Ammo – a Comparison

Many firearms novices, and even seasoned shooters, are unsure of the difference between .223 and 5.56 mm ammo. Many incorrectly assume that they are interchangeable, but they are, in fact, not interchangeable.

Here are a just a few of the similarities and differences between .223 and 5.56 mm ammo as well as a little history on these two types of bullets.

The 5.56 x 45 mm round first came about during the late 1950s as an experimental cartridge for the U.S. Military M-16 rifle. This “small caliber, high velocity” round was desirable because it helped overcome weight and control issues present in the 7.62 x 51 mm round that was used in the previous-generation M-14 rifle.

After the official adoption of the 5.56 x 45 mm round as the NATO standard in 1963, Remington introduced a .223 caliber “civilian” version of the round shortly thereafter in 1964.

A very popular sporting rifle for today’s enthusiasts is the AR-15. This firearm is commonly chambered for 5.56 mm ammo because it is a descendant of the M-16 rifle.

Case Size
The outer dimensions of the brass case for the 5.56 x 45 mm and the .223 are the same. However, there are many other factors that determine what is a safe round to chamber in a particular firearm, and although the case size is identical on these two rounds they are not interchangeable.

The 5.56 mm chamber has a longer distance between the mouth of the cartridge and the point at which the rifling engages the bullet. The .223 Remington chambering has a shorter distance before the rifling engages the bullet. This means that the pressure within the chamber that is produced when firing can be dramatically different for these two rounds.

Due to the differences in the chamber described above, a .223 round fired through a 5.56 mm chamber will result in lower pressure than what is safe for the firearm and is therefore typically safe. However, a 5.56 mm round fired through a .223 chamber will result in higher pressure than what is safe for the firearm and thus is not considered safe. This over-pressuring of the chamber can lead to all sorts of dangerous malfunctions and should be avoided at all costs.

Bottom Line?
It is safe to fire a .223 round from most 5.56 mm firearms, but it is not safe to fire 5.56 mm rounds from a .223 firearm. Because these two rounds look very similar and have the same size case, always check the head stamping on your ammunition before loading your firearm just to be safe.

Which is Best For You?
There are all sorts of considerations that go into deciding what caliber of firearm to purchase. Your shooting style, what type of shooting you prefer and many other variables will impact the decision about which caliber is right for you.

Please stop by the shop and chat with one of our friendly team members if you have questions. We stock all types of ammo, including .223 and 5.56 mm in bulk packs and smaller boxes of 20 rounds as well. Our team looks forward to meeting you and chatting with you about what caliber round and type of firearm will be the best fit!


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