Selecting an AR Class
Short Story: The class of AR that you want depends on how you want to use it.
The first step in designing your AR is to select a class of AR. Your options are generally as follows:
- Pistol Caliber AR
- Small Frame AR
- Large Frame AR
Each of these has vastly different capabilities, benefits, and drawbacks. We will evaluate each class by the following characteristics:
- Ammo Availability: While popularity can impact availability at times, we focus on the number of rounds produced domestically per year. In the case of the three default cartridges, all enjoy widespread popularity and availability in good times.
- Muzzle Velocity: Muzzle velocity generally translates into better external ballistics. Remember that acceleration toward the ground due to gravity is a constant 9.81 meters per second per second; each second that the bullet is in the air, it is travelling 9.81 m/s faster toward the ground than it was one second prior. If a bullet can cover the same distance in less time, it drops less over the same distance (less time in air means less downward acceleration and less drop).
- Muzzle Energy: Velocity, alone, does not mean a bullet can travel farther. The lighter a bullet, the less momentum (Newton’s Second Law) and the easier things like air molecules, humidity, and wind will be able to slow it down. Muzzle energy combines velocity and mass to give a normalized representation of the kinetic energy of a bullet as it exits the barrel.
- Effective Range: Ignoring extrinsic factors, effective range is the practical result of bullet shape/stability (which determines how well it flies through the air), bullet weight/density, and velocity. As a bullet travels through the air, it loses speed and, therefore, kinetic energy. It may also lose axial stability and begin tumbling, but we will not consider this in our evaluation (due to variation from bullet to bullet). We consider a bullet with at least 200 ft. lbs. of kinetic energy to have enough energy to do sufficient damage to a soft target. For ethical reasons, please note that we DO NOT recommend anything less than 1,000 ft. lbs. of energy for hunting medium/large game.
- Penetration: Penetration is a consequence of kinetic energy and bullet shape. The heavier and faster the bullet (i.e. higher kinetic energy), the greater the penetration. The “pointier” the bullet, the greater the penetration. Because every bullet loses different amounts of kinetic energy at different ranges, we compare penetration at short distances.
- Recoil: Recoil impacts the shooter in negative ways. The greater the recoil, the less pleasant the shooting experience. The greater the recoil, the longer it takes to reacquire your sight picture and re-engage a target after a shot.
- Compactness: The more compact a cartridge, the less space it occupies (obviously) and the more portable it is. So, the more compact the cartridge, the more of it you can carry in the same amount of space.
- Ammo Weight: As a complement to compactness, the weight of ammunition is also a contributor to portability. In most cases, a bullet will incapacitate a target within the effective range, regardless of the size of the cartridge. Having more rounds available can mean more opportunities to hit your target. More rounds of the same cartridge equates to more overall weight. The same number of rounds of different cartridges can also impact the overall weight. And a few extra pounds of gear can make a big difference in the field. Suffice to say that in most cases, the more rounds you can carry for the same collective weight, the better.
- Ammo Affordability: All else being equal, the less you have to pay for quality ammunition, the more you can shoot or store on the same budget.
The table below summarizes our comparison of the AR classes:
| Attribute |
Pistol1 |
Small Frame Rifle2 |
Large Frame Rifle3 |
|---|---|---|---|
| Ammo Availability | +++ | +++ | +++ |
| Muzzle Velocity (16" barrel) | 1,295 fps | 3,100 fps | 2,800 fps |
| Muzzle Energy (16" barrel) | 428 ft. lbs. | 1,174 ft. lbs. | 2,559 ft. lbs. |
| Effective Range (>200 ft. lbs.) | <200 yds. | 600-700 yds. | 1,800-1,900 yds. |
| Penetration | + | ++ | +++ |
| Recoil | +++ | ++ | + |
| Compactness | +++ | ++ | + |
| Ammo Weight | +++ | ++ | + |
| Ammo Affordability | +++ | ++ | + |
1: 9mm Luger (Federal 115gr JHP)
2: 5.56x45mm NATO/223 Remington (Federal 55gr FMJBT 5.56x45mm)
3: 7.62x51mm NATO/308 Winchester (Federal 147gr FMJBT 308 Win)
Pistol Caliber AR (i.e. AR-9)
The pistol caliber AR, often referred to as a pistol caliber carbine, capitalizes on the portability of pistol cartridges. These firearms typically come in one of the popular pistol cartridges, like 9mm Luger, 40 S&W, 45 ACP, and 10mm Auto.
Benefits
- Higher velocity than in a standard pistol frame. Because the barrel of a pistol caliber AR is likely to be longer than that of a standard pistol, the gas produced by the burning powder has longer to act on the bullet, which increases the velocity of the bullet.
- Higher muzzle energy than in a standard pistol frame. The increase in velocity also increases the muzzle energy, which means a pistol cartridge fired out of a longer barrel has more knockdown power than one fired from a shorter barrel.
- Longer range than in a standard pistol frame. The increase in muzzle velocity means that the bullet travels farther per unit of time (obviously). This reduces the effect of gravity and generally equates to longer range capabilities.
- Lighter recoil than rifle cartridges. Pistol cartridges have less driving force compared to rifle cartridges. Less power forward means less recoil rearward (Newton’s Third Law). This contributes to a more pleasant shooting experience and faster follow-up shots.
- Smaller and lighter than rifle cartridges. Pistol cartridges are more compact and lightweight than their rifle caliber counterparts. This means you can carry more rounds in less space and for less weight.
- Cheaper than rifle cartridges. Pistol ammo is generally a fraction of the cost per round when compared to rifle ammo. You will pay around $0.20 per round of 9mm Luger (40% of the cost of 5.56 NATO and 20% the cost of .308 Winchester).
Drawbacks
- Lower velocity than rifle cartridges. Pistol cartridges are designed for close quarters. The ratio of the hunk of metal vs. the powder charge results in a lower velocity projectile when compared to a rifle cartridge. For example, a 115gr 9mm Luger bullet exits the muzzle at less than half the velocity of a 55gr 5.56mm bullet when fired out of a comparable barrel. This has a dramatic impact on external ballistics, especially beyond close quarters.
- Lower muzzle energy than rifle cartridges. Despite the heavier bullet, the lower velocity of a pistol cartridge results in significantly less muzzle energy than a rifle bullet.
- Shorter range than rifle cartridges. Most pistol cartridges are designed for short range knockdown (heavy bullet and high weight-to-powder ratio) and compactness (short and squat). Because of the shape of the bullet, the ballistic coefficient (the measure of how easily it moves through the air) of most pistol cartridges is absolute garbage. They bleed off speed and energy much faster than a rifle bullet would. Given that they start off with a much lower muzzle energy than a rifle cartridge, this ballistic performance results in a much shorter effective range.
- Less penetration than rifle cartridges. Because of the shape of most pistol bullets, you will not get the same penetration as with a rifle cartridge, even if muzzle velocity and energy were equivalent.
Best Application
The best place for a pistol caliber AR is for close quarters work (because you don’t need a rifle cartridge to shoot something that’s 10 feet away) and for recreational shooting (because pistol ammo is cheaper).
Small Frame AR (i.e. AR-15)
This is the AR-15. If you don’t own an AR-15, this is where you should start. ARs in this class come in a variety of calibers, though the default caliber is 5.56 NATO. Additional common calibers include, .22 LR, .300 AAC Blackout, .224 Valkyrie, 6.5 Grendel, and 6.8 SPC.
Benefits
- Ubiquitous cartridge. The 5.56 NATO is the default cartridge of the U.S. infantry forces. From a round count perspective, it is probably the most common rifle cartridge in the U.S.
- Higher velocity than pistol cartridges. Compared to the pistol caliber AR, the velocity is much higher (5.56 NATO has more than double the velocity of 9mm Luger). This translates to numerous benefits, ballistically.
- Higher muzzle energy than pistol cartridges.
- Better range than pistol cartridges.
- Lighter recoil than large frame rifle cartridges.
- Better penetration than pistol cartridges.
- More compact and lighter than large frame rifle cartridges.
- Cheaper than large frame rifle cartridges.
Drawbacks
- Lower muzzle energy than large frame rifle cartridges.
- Heavier than pistol cartridges.
- Shorter range than large frame rifle cartridges.
- Slightly heavier recoil than pistol cartridges.
- Lower penetration than large frame rifle cartridges.
- Less compact and heavier than pistol cartridges.
- More expensive than pistol cartridges. You will pay around $0.50 per round for 5.56 NATO (2.5x the cost of 9mm Luger).
Best Application
This is the best all-around platform for most applications. With intermediate range, recoil, size, and weight, you can do a lot with this class. You won’t be knocking things down at a mile, but you’ll be able to engage targets far enough away to give yourself time to set up a shot.
If you don’t own an AR, this is the place to start.
Large Frame AR (i.e. AR-10/LR308)
This is the AR-10 (or more appropriately for our build, the LR308). It is a beefed up version of the AR-15 and brings the benefits of a larger cartridge to the table. By default, this class is chambered in .308 Winchester, though the 6.5mm Creedmoor is a better cartridge for long range applications.
Benefits
- Higher velocity than pistol cartridges.
- Higher muzzle energy than pistol cartridges and small frame rifle cartridges.
- Better range than pistol cartridges and small frame rifle cartridges.
- Better penetration than pistol cartridges and small frame rifle cartridges.
Drawbacks
- Heavier recoil than pistol cartridges and small frame rifle cartridges. This has an effect on the overall shooting experience and speed of follow up shots. It also has a cumulative fatiguing effect on the shooter.
- Less compact and heavier than pistol cartridges and small frame rifle cartridges. You will also be carrying a heavier weapon, in addition to heavier and bulkier ammo.
- More expensive than pistol cartridges and small frame rifle cartridges. You will pay about $1.00 per round for .308 Winchester (5x the cost of 9mm Luger; 2x the cost of 5.56 NATO).
Best Application
The AR-10/LR308 is best suited for longer engagement distances and support against harder targets. Because the firearm and an equivalent number of rounds are substantially heavier and larger than the other classes, this is the least portable option and is probably not the thing you want to be clearing buildings with. But when you need extra power and are engaging targets at a longer distance, the benefits outweigh the size and weight.
If cost is a factor, this class is the most expensive option to build and shoot.