Effective Range of the 5.56 / .223 FMJ cartridge
After having several discussions with friends I did some research on the effective range of AR-15 rifles using various types of ammunition. In this post I'm going to discuss the maximum effective range of the most common bullet weight and type, 55gr full metal jacket (FMJ). As many people know the standard 55gr full metal jacket projectile's primary wounding capability comes from fragmentation. There is wounding performed by hydrostatic shock as well, and of course the path of the bullet. If a bullet itself hits a major blood vessel or vital organ it can of course be lethal. But when a bullet expands or fragments it greatly increases both the wound channel and the potential to damage something vital. It's a morbid topic, but if you use a firearm for hunting or defense, you want it to be effective, so it's an important discussion.
If a person chooses 55gr .223 or 5.56 NATO ammo for use in a defensive carbine it's important to fully understand the fragmentation qualities and understand what the maximum distance is at which a bullet fired from your rifle will still possess fragmentation qualities. This is because it takes a fair bit of velocity to give the bullet enough energy to fragment when it strikes a target. Most experts suggest the necessary velocity to ensure reliable fragmentation is 2700 fps. Since I've read that the bullet can fragment down to 2500 fps, although not as explosively as at higher velocities, I'm going to split the difference and choose 2600 FPS as the threshold. I've chosen this bullet for this discussion because by a large amount it's the most common bullet found in .223 and 5.56 ammo.
Barrel length is correlative to muzzle velocity. The shorter the barrel, the lower the muzzle velocity. The shortest barrel seen in 5.56 rifles is 7", which is pretty stupid honestly. A large percentage of the powder burns outside of the barrel and the velocity is way down when compared against a standard length carbine. For purposes of this blog, we'll be starting at a barrel length of 10.5", a popular length for short barrel rifles used by military, police and civilians.
A M193 (the military standard 55gr 5.56 cartridge) generates a velocity around 2750 fps in a 10.5" barrel AR-15. This isn't much higher than the fragmentation velocity. Since the bullet will be losing speed every moment after it leaves the barrel, we can figure out it's going to drop below the magic velocity threshold around 40 meters. That's not much of a range at all. If you are using a 10.5" rifle for defensive use, you'd likely be better off with a different type of bullet.
As an aside, I'm using nice round numbers here, if you want to know exact ranges you'll have to know your exact velocity out of your rifle. It could easily be 50 fps plus or minus the average velocities listed here which will change the range by a meter or ten.
If we move up to an 11.5" barrel we pick up a bit more velocity, 2875 fps or so. It's a pretty nice bump from 10.5". It shows how a cartridge and the burn rate of the powder chosen for factory loadings in that cartridge is tuned to a specific barrel length. So by moving from a 10.5" to a 11.5" barrel we extend our fragmentation range out to 70 meters. This is a more useful range, but still very limited.
12.5" gets us to 2950 fps. This brings the range to 85 meters, still very limited.
14.5" gets us 3050. This is the barrel length used by the military in the newest version of the M-16 rifle which is commonly called the M4. The velocity increases are slowing down at this point, most of the powder has burned by the time the bullet exits the barrel. This takes our effective frag range to 100 meters. This is becoming more practical for an effective defensive range.
16", the most common barrel length sold in AR-15 carbines and the shortest length available for general sale, gives us a muzzle velocity of of 3125. Fragmentation range extends out to just over 110 meters, not much longer than the 14.5" barrel.
The second most common barrel length for AR-15 rifles is 20". The M193 round gets 3250 fps and a range of just over 150 meters. This gives us a 50% increase in effective range over the 16" barrel.
24" gives us 3300 fps, barely any more velocity than than the 20" barrel. We've passed the sweet spot for this particular cartridge, where additional barrel length gives us lower velocity increases. 16 to 20 inches is really the ideal barrel length for this cartridge. A 24" barrel gives us very little fragmentation distance over a 20" barrel.
All of these calculations were done around the military spec M193 5.56 NATO cartridge. If instead you use .223 Remington ammo offered by Winchester, Remington, PMC or other companies, your velocity could be 100 fps lower than M193, as .223 is loaded to a lower pressure than 5.56 NATO. If you are using Wolf, Brown Bear or Silver Bear .223 ammo, your velocities could be 150 fps slower. That can reduce effective fragmentation ranges effectively. Using steel cased Wolf .223 ammo in a 11.5" short barrel rifle gives almost no fragmentation capability.
In no means is this an attempt to talk people out of using 55gr ball ammunition for defense. It is an ideal cartridge for defensive work, especially in residential areas where light obstacles (like interior walls) will cause it to fragment very quickly and reduce the chance of missed shots striking or hurting unintended targets. But if it is the ammo you chose to use, you should understand it's capability. Once you extend past it's fragmentation range, you are likely better off using another ammunition type. For instance, the 55gr TSX bullet made by Barnes should offer reliable expansion down to 1,800 fps. This extends the range quite a bit at which it offers good terminal ballistics. A reasonable person could practice with 55gr FMJ ammo and use the TSX bullet for hunting or defense to extend your effective range.
Fortunately, there are scores of options for 5.56 and .223 ammunition, with many bullet types designed for different purposes. Some bullets are designed for varmints and offer very fast expansion, some for medium game and expand and retain weight and offer deep penetration. Choose your ammunition wisely for your purpose.
If you'd like to see ballistic gelatin testing of various .223 and 5.56 rounds, I recommend the Brass Fetcher video channel.
This is a repost with an addition from my older blog found here:
http://angry-peasants.blogspot.com/2012/08/effective-range-of-your-223-or-556-rifle.html#more