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AR-15 Ammo and Bullet Sizes: Complete Guide

AR-15 Ammo and Bullet Sizes: Complete Guide
October 4, 2022 Edited March 28, 2023 13985 view(s)
AR-15 Ammo and Bullet Sizes: Complete Guide

Shopping for ammunition can get confusing fast. For example, what’s the difference between .223 Remington and 5.56x45mm NATO? Do you want a full metal jacket, hollow point, boat tail, soft point, or something else? Does the green tip make a difference? Once you roll bullet sizes in the mix, then it’s really confusing. 

For this article’s sake, we’ll focus on the standard .223/5.56x45mm AR-15 rifle and pistol chamberings. So let’s dive in and discuss AR-15 bullet sizes. 

AR-15 calibers lined up side by side on workbench


What Are the Common Bullet Sizes?

There are a few bullet sizes that are among the most common bullet sizes. First and foremost, when we talk about ‘bullet size,’ we describe the weight of the bullet in grains. But more on that a little later. 

Also, there is another factor that plays heavily into AR-15 bullet sizes: barrel twist. AR-15s are built with a few standard barrel twists for .223 bullets. The typical rates of twist are 1:7, 1:8, and 1:9. 

The most common weights are 55-grain, 62-grain, and 77-grain bullets. 55-grain bullets are standard bullet weights used in FMJ bullets for target shooting and training. It is an excellent all-around weight, and the best twist rate for a light bullet is 1:9. Lighter bullets perform the best coming out of a slower twist rate, while heavier bullets prefer a faster spin. 

5.56 Ammo box on workbench


What Do the Numbers Mean?

The numbers you’ll find on a bullet size chart reference the weight of the bullet. For all practical purposes, this is the number that matters most for AR-15 shooters. 

All bullets and gunpowder are measured in grains. Bullets in .223 caliber are comparatively light due to their small diameter. A standard 7.62x39mm weighs 123-grains, and the Winchester/.30-06 Springfield uses bullets ranging from 150-grains to over 200-grains. Bullets in .223 are as light as 40-grains, with some bullets weighing up to 85-grains.


What Is the Weight Measurement Used for Bullets? 

All bullets and gunpowder are weighed in grains. However, it is an obsolescent weight measurement and isn’t typically used in any other application. 

For whatever reason, it remains the standard for ammunition. Your bullet size chart lists bullet weights in grains. It isn’t up for discussion. The entire industry is built around grains and will always be built around grains. 

Here’s the deal: 

Contrary to popular belief, using a barrel twist that isn’t optimal for the weight of ammunition will not send it off on some wild tangent. It only means it’s less than ideal. It will still cycle fine, shoot fine, and be accurate; just not quite optimal. 


Does the Type of Bullet Change the Size?

This question gets tossed around a lot: does the type of bullet change the size? It certainly can, but it doesn’t necessarily have to. 

An FMJ bullet is typically 55-grains; this has been the standard for many years. However, you may find other FMJs a little heavier; the rules are not all that hard and fast. 

The longer the bullet, the heavier the weight. Therefore, precision shooters or other long-range applications (e.g., Prairie Dog hunters) generally opt for a heavier bullet; heavier bullets are less prone to wind drift, making them the best options for those long shots. 

Oddly enough, some factors don’t affect bullet weight much as you think. For instance, hollow point (HP) bullets in .223 are usually the same weight as a similar FMJ, with 55-grain being the norm. 

Heavy AR-15 bullet sizes (69-grain, 75-grain, and 77-grain are the most common) are usually match-grade ammunition. These are usually fired out of long barrels with a faster twist. Interestingly,223 operates best from barrels 24” long or less; it is not well suited for extra-long barrels. This is a feature, not a bug. The .223 was tailor-made for gas-operated rifles and optimized for the 16” barrel of the original Stoner design. Of course, a barrel longer than 24” doesn’t hurt anything, but there is also no value added, just weight and heft.


What Is the Ideal Bullet Size for an AR-15?

Now, this is a loaded question; like all calibers, there is no ideal bullet size. Instead, bullet sizes are tailored from application to application. So, for example, you wouldn’t use the same bullet weight in your .30-06 hunting rifle to bag a meat doe as you would hunt a bull elk; a 150-grain is adequate for the doe, but you’ll want 220-grain for the bull. 

This analogy applies to all bullets and weights. Typically, defensive loads in .223 are lighter, in the 55- to 62-grain range. These have proven adequate over decades of service in the M16 and M4 carbines. However, if you use the .223 for feral hogs (note: please use a larger caliber; we have plenty of options!), you’ll definitely want a heavy bullet made specifically for dangerous game.


What Size of Bullet Should You Use in Your Rifle?

Truth is, we can’t tell you what size of bullet you should use; it depends entirely on what you do with your rifle.

If you have a ranch rifle or truck gun for vermin and livestock predators, a loadout of 55-grain HP is a great option. Pair those with a rifle with a 1:9 twist barrel; you’ve got a stone-cold coyote killer. This is also a good pairing if you are worried about two-legged predators. 

Are you planning on shooting heavier bullet grains? You might want to slide down to a 1:8 twist if you’re in the mid-weight range (62- to 77-grain), and if you shoot heavy slug ofter, grab a rifle or an upper with a 1:7 twist barrel

AR twist Chart


What About an AR Pistol?

Our AR-15 pistols use a 1:7 twist barrel to best compensate for their short length. For AR-pistols, you should use heavy bullet sizes to optimize energy carriage over a distance; light bullet sizes lose their energy rapidly from a short barrel. Therefore, you should consider going with bullets weighing anywhere from 69-grains to 80-grains for your AR pistol or short-barreled rifle (SBR).


Other Common AR-15 Calibers

What makes the AR-15 so popular? One, easy operations and maintenance. But more than that is the infinite modularity. With two simple takedown pins, you can swap out an entire upper receiver with another caliber for different uses. 

Besides the classic .223/5.56x45, the most popular AR-15 calibers are:

We carry barrels, upper receivers, and whole rifles for these calibers. Each has unique characteristics and uses, requiring many different bullet types and sizes. For instance, the .350 Legend was designed for states with straight-wall caliber requirements for deer hunting. Same with the .450 Bushmaster. Apparently, the 12.7x42mm was designed to either split engine blocks or stop locomotives in their tracks.


Wrapping It Up

The ammunition world is complex and takes time and experience to understand. For example, AR-15 bullet sizes generally range from 40-grains to 80-grains on the high side, but this all depends on your application, barrel twist, and barrel length. There is nothing wrong or dangerous about using bullet sizes that are heavier or lighter than optimal for your barrel twist but be warned that they won’t perform to their full potential. 

Bear Creek Arsenal is your source for barrels, uppers, rifles, and accessories for whatever bullet size you prefer, along with every other common AR-pattern caliber (and a few not-so-common calibers!). We offer some of the most accurate rifles at rock-bottom prices, so don’t hesitate; check out our selection now and own the range!


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Paul Smigiel
October 18, 2022
Thanks for a very informative article. Now you've raised more questions for me to find answers to. I'll be looking to you for answers in the very near future. Thanks
Hector E Soto
October 27, 2022
AK and AR's are highly overrated and overly priced. Firearm companies must invent a new rifle and PCC platform that is not an AR or AR platform
Charles B Williams
February 12, 2023
Will you make a left hand 6.5 Grendel again
Andrew Cowan
May 7, 2023
Just a thought but can or do you offer an AR platform in .243 Win? It seems logical and is something that I would be interested in. It’s a very overlooked caliber in my opinion and offers some advantages over the smaller calibers. I suppose that the 6.5 creed may out perform it but I tend to lean toward the tried and true things. I’ve never seen or heard of one but it seems like a fairly simple adaptation from the AR 10. Thanks for taking my question and I look forward to you reply.
May 8, 2023
We do offer .243 Win in the AR 10 here:
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