Noobie guide to 5.56 AR rifle ammunition.

Noobie guide to 5.56 AR rifle ammunition.

When I was first learning about AR 5.56 chambered rifles, there was a lot of confusion about what ammo to buy. This is the guide I wish I had.

First off, if you are training or bought a rifle – GO READ THE MANUAL FOR THE RIFLE. Hopefully you did research on what you were buying before you went to the store. Now you need to go read the manual for your rifle.

So, now it’s time for practice and training. Just like a car, you should find a certified instructor and a safe place to practice. Finally, you need to buy the fuel for your car: the ammunition. Many training programs ask you to provide your own ammunition even if they are providing the rifle. So imagine my surprise when I found LOTS of confusion about what to buy for 5.56 chambered AR rifles. Lets go through them one at a time.

Topic 1: 5.56 vs Remington .223:

In purchasing ammunition, you may be told/see people selling .223 ammunition for your 5.56 weapon. You’ll see forums where people will tell you one works in the other, others say you cannot. Adding to the problem is that Remington .223 (also called 223 Rem or just 223) and 5.56 rounds will usually fit in either rifle. Some people will swear by and even fire 5.56 rounds in a weapon chambered for .223 Rem and vice versa. So what’s the difference?

To be clear: 5.56 ammunition and rifle chambers are not the same as Remington .223 ammunition and rifle chambers. There are combinations in which where one kind of ammunition can be used in the other rifle, but there are enough differences that you may experience feed or accuracy issues.

The summary is this: 5.56 rounds generate much higher chamber pressures than Rem .223 rounds. 5.56 weapons also have very slight differences in the chamber dimensions and are designed to withstand the higher pressures of 5.56 rounds. .223 Rem weapons have chambers that are NOT designed for the higher 5.56 pressures and have slightly different dimensions that match .223 rounds. The difficulty is that the differences are small enough that either round will fit, and usually fire, in either weapon.
Because 5.56 chambers can withstand higher pressures, it is safe to fire .223 Rem cartridges in any gun chambered for 5.56 – but you might experience feed issues. It is never considered safe to fire 5.56 cartridges in a firearm chambered for .223 Rem. Sure – it might work hundreds of times or even for years, but eventually, the extra pressure stress of 5.56 rounds in a .223 Rem chambered rifle may cause feed issues all the way to catastrophic failures (chambers that explode when you fire them).

Sorting .223 vs 5.56 -
Check your head stamps – Remington 223 on the left, and 5.56 NATO on the right. Do NOT put 5.56 NATO into a gun chambered for 223 Rem – but the reverse can be done.

The actual technical differences between the rounds is quite fascinating if you’re interested. It is especially important if you’re interest in reloading your own rounds. Go read tons of more technical details here or see this video here.

Always check the stamps on the end of the round and make sure you’re putting the right ammo in the right weapon.

Topic 2: 5.56 vs 5.56x45mm vs 5.56 NATO

Now for some terminology confusion. Is 5.56 the same as 5.56x45mm, 5.56 NATO, or 5.56x45mm NATO? Ugh – it was so confusing. Many times it just seemed sloppiness in the description, but other times there are real differences.

Summary: 5.56, 5.56x45mm, 5.56 NATO, and 5.56x45mm NATO are not always the same round – though for practical purposes they are and will fire in a weapon designated as 5.56. 5.56 is usually just a shortcut for 5.56x45mm. There are no 5.56 rounds that are other than x45mm that I have run across. So, 5.56, 5.56×45, and 5.56x45mm are the same designation.

Having a NATO stamp and not, however, does indicate a real difference. The NATO stamp is an indication that the NATO spec has been followed when making the round. An official 5.56 NATO round will have a little cross in a circle stamped on it:

Note the + in circle headstamp to ensure it is a 5.56 NATO round.

However, you might run into rounds without the NATO stamp but say 5.56 such as these rounds from Prvi Partizan:


Having a NATO stamp or not often has to do with how primers are seated and other technical facts about how the round is made – but they should work in your 5.56x45mm chambered rifle. Personally, I only stick with the 5.56x45mm NATO rounds. For me, consistency of quality in the round is more important than saving a few dollars on a box. Rounds without the NATO stamp might indicate lower quality standards and have lower tolerances – so use your discretion.

The takeaway is that 5.56, 5.56×45, and with/without the NATO designation are designed to work in your 5.56 chambered rifle.

Topic 3: Grains: 40, 55, 62, 77, and 80 Grain Rounds

Next up, there is also the weight and composition of the round. Lets start with grain. What’s the difference between 5.56x45mm 55 grain, 40 grain, 77 grain, and other grain amounts? And what about those 62 grain penetrator/green-tip rounds?

People argue endlessly about this topic. Some argue heavier rounds have more ‘stopping power’; others argue about overpenetration, accuracy at distance, etc. The most important part is to get the right one for your weapon – and the right one for your shooting goal. In short, different weight bullets have different performance characteristics.

First, an anatomy and terminology lesson. The term ’round’ or ‘cartridge’ refers to the entire package: its casing, powder/propellant, rim, primer, and bullet. Ammunition is the plural of round/cartridge. A ‘bullet’ only refers to the solid projectile (tip) propelled from the end of the cartridge when the gun is fired:


The bullet part is measured in a unit of mass called grains (abbreviated “gr.”). One pound is equal to 7000 grains, and there are 437.5 grains in an ounce. This is the metal projectile part of the bullet that leaves the barrel while the casing pops out and onto the ground.

5.56 rounds usually are in the 40-80 grain range. 9mm handgun rounds are in the 115-140 grain range. A 50mm BMG round is a monstrous 750 grains. The most common 5.56x45mm round is 55gr. A 40gr bullet weighs less, a 70 gr bullet weighs more. So why are there heavier or lighter ones – and should I pick one?

You read the manual as first indicated – right? 🙂 Did it mention anything about round weight? Or perhaps you noticed a barrel twist amount? I’m going to do a mediocre job summarizing this topic – so go read more later – but this should get you going.

One of the great accuracy improvements in early firearm design was the creation of groves INSIDE the barrel where the bullet travels. As the bullet travels down the rifle, the groves start the bullet spinning – and this rotational spin creates stability in flight – which translates into accuracy at distance.

Twist rates as a ratio – smaller ratios as more rotations

There have been whole fields of study done on barrel length and twist rates along with how projectile size, shape, weight, and length affects accuracy, power transfer, and various other performance characteristics. Depending on your target distance and goals, you might choose different combinations of round and barrel. A marksman hitting targets at 800 yards has very different requirements of his rifle and bullet than a SWAT team clearing rooms in a tight building.

Most rifles have barrel lengths and twist rates that make certain rounds give the best performance. While you might hear that the bullet weight dictates the twist rate (or vice versa), this is not entirely true – but usually true enough for our purposes here. Another common mistake is saying that the weight in grains has to do with the amount of powder in the round. The reality is a heavier grain round might have more powder or it might not. From a scientific point of view when comparing bullets and twist rates – length of the bullet has more to do with the best twist rate than weight alone. Heavier bullets are often longer in length to achieve the extra weight – so weight is a pretty good practical measure in most cases. Still, this is a good reminder to be cautious and read solid research papers when reading about topics like twist rates, weights, accuracy, and other performance characteristics because many online ‘experts’ dangerously over-simplify.

At any rate, a simplified chart for 5.56 twist rates to round weight looks something like this:

Ideal Bullet Weight vs Twist, Shooters Log

So what happens if I mismatch the weight and twist rate? It won’t damage your rifle, misfire, or jam. Instead, all that happens is your accuracy usually goes down. If you fire a 62 grain round in a 1:9 twist rate barrel (as opposed to a 1:8 or 1:7 twist rate barrel), the only difference you will likely notice is that it is slightly less accurate than shooting a 55 grain round in a 1:9 twist rate rifle.

All of these rounds will work perfectly safely in any twist rate barrel. You can put any weight bullet in any twist rate barrel – but mismatching them will result in lower/more inconsistent accuracy.

Summary: For most AR platforms with standard twist rate barrels, a 55 grain bullet is what you want. Consult your manual. The best advice says to start with what is recommended for your rifle until you are proficient enough to know what you’re doing to choose otherwise.

So what about those 62 grain penetrator rounds? This leads us naturally into the next topic.

Topic 4: Full metal jacket, penetrator, frangible and other rounds

Now we get into bullet composition. What is that little bit of metal that flies out of my barrel made of and why does it matter? Again, you’re going to run into MASSIVE amounts of people talking about all kinds of exotic rounds and why you should be using them. Lets stick with the basics.

Full metal jacket or Ball: Sounds scary and imposing – but it’s really the most common and standard kind of round. A full metal jacket round (FMJ) is just that – a bullet made of a soft core (often lead) encased in an outer shell (“jacket”) of harder metal. In military nomenclature, the full metal jacket round is often labeled ball ammunition. 5.56 often has a uniform lead core with a copper/copper-nickel jacket or shell. A bullet that has a jacket generally allows for higher muzzle velocities than bare lead without depositing significant amounts of metal or damaging the bores from steel or armor-piercing core materials.

FMJ Bullets - Fog Ammo

Summary: the full metal jacket rounds is your bread and butter ammunition for your 5.56 rifle. They’re designed to shoot it above all other types. This is almost certainly what you should buy for the range or practice as you learn. It’s often the cheapest and easiest to buy in bulk.

Penetrator or Green Tip: You might see ‘green tip’, perpetrator, or M855 62gr 5.56x45mm rounds. What about them?

These rounds differ from the standard ball or full metal jacket rounds in that they often have a hybrid core designed to penetrate targets better. A full metal jacket core is usually uniform material inside the metal jacket. But as you can see below, penetrator rounds are often a mix of traditional soft material and a steel perpetrator part.

M855 Cutaway,

Will these fire safely in your 5.56 and can I use them every day? Yes. However, depending on your barrel length, twist rate, and other factors – these often turn out to be not as accurate or consistent as full metal jacket rounds on many stock rifles that have 1:9 twist rate barrels. Again, a 1:9 twist rate barrel is best suited for 55gr ammo and may be slightly less accurate shooting 62gr rounds. There’s also some evidence that hybrid core rounds can never be as balanced as bullets with uniform cores – which results in less accurate flight. Here’s some further info and we’ll talk more on that in Paul Harrell’s video below.

Hollow point, Jacketed Hollow Point, and Soft Point:

What about the geometry of the bullet itself? Turns out, there’s also different tip styles. Instead of more penetration, you might want LESS penetration in some cases.

FMJ Bullets - Fog Ammo
Full metal jacket rounds

Full metal jacket/penetrator rounds have a copper jacket that completely covers the round. They tend to have high accuracy and hit targets while maintaining their velocity. While this is great for range and target shooting because it offers a consistent performance when training – in real situations it often means the bullet exits the back of the target at high speed and continuing on to hit further targets.

Hollow Point Bullets - Fog Ammo
Hollow point rounds

Hollow point and soft point rounds are designed to stop inside their target. It does this by expending their energy into the target. It does this by opening up like a mushroom as soon as it hits a target. It’s much like a fist. A fist is easy to push through water, but open your hand and it gets much harder. The energy gets expended into the target instead of just traveling through it. This is a desirable trait if you are using the rounds for personal defense and hunting.

Hollow point rounds get their name from the fact they have a tip that has a hollow space. This means when the hits a target, it immediately expands like opening an umbrella the wrong way in a wind storm.

Soft Point Bullets - Fog Ammo
Soft point rounds

Soft point rounds are as they are described, the points often expose the soft lead core. The soft tip expands on hitting the target, but not as much as hollow points. These are good for large game that have tough, thicker outer hides.

Will these rounds work in your 5.56? Yes – without issue. Just pay attention to their weight in grains and recommendations. Also be ready to pay more and only buy in small quantities. These rounds become highly specialized for their purpose and often sell in small quantity boxes for home defense.

Summary: If you’re going to the range to practice, it is probably best to stick with full metal jacket since they are cheapest and don’t need the specialized features of hollow and soft points.
Read more here.

Putting it all together:

Paul Harrell has a great YouTube channel. One of the things I like about him is that he’s got an great amount of military/police training and knowledge. He also backs it up with actual, very practical, demonstrations. Here he does a comparison of a standard 5.56 NATO XM193 55gr round and a M855 62gr perpetrator round.

You should now be able to combine everything we’ve learned and understand what he’s talking about. And learn a little about how to listen critically to many online ‘experts’.


Do NOT consider these definitive or completely accurate. I used them for reference of various bits and pictures I needed.

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