AR Barrels and Projectiles 101

Here is a link to a great article.

Though some of these are touched on in the article, I'll go a bit further on some frequently asked questions. Here are some of my general knowledge points about the AR15 platform, and what barrels go with what bullets:

LENGTH

16" Barrels
This is broadly true for 14.5" barrels with permanently attached 1.5" flash hiders as well as full 16" barrels (regardless of flash hider).
- The projectile will have a reduced velocity when compared to a longer barrel length, but the round can still be fired accurately out to 600+ meters.

20" Barrels
- Higher velocity than the carbine length barrels.
- Barrel construction being the same, these will be heavier than a carbine counterpart.
- Higher velocity means better performance at longer distance
- 24" barrels will produce even more velocity, but on the .223 / 5.56 round, the increase is marginal.

TWIST

1:9 Twist Barrels
16" - These barrels will fire projectiles up to 69 grains in weight without problems.
20" - Can fire up to 75 grain projectiles without problems.

1:7 Twist Barrels
- In any length, these can generally fire up to 90 grain projectiles... which is about as heavy as you can find for .223 / 5.56


1:8 Twist Barrels
- Most often used by specialists for superb accuracy.
- Can handle up to 85 (sometimes 90) grain projectiles without a problem.
- Usually found in 20" and 24" varieties - not as common in 18" or 16" models.

PROFILE

Thin profile barrels
- Also include medium profile in this group
- Can be accurate
- Do not tolerate heat very well: accuracy degrades more quickly than a thicker barrel.

Heavy Barrel
- Also include special fluted barrels in this category.
- Generally more accurate, due to less barrel whip
- Heavier than a slim profile or government profile

Government Profile
- Often dubbed "M4 Profile" or "A4 Profile"
- Part of the barrel is slimmer for weight reduction, part of the barrel is thicker for better heat resistance.
- This barrel sees the worst of both worlds - getting the barrel whip of a thin profile, but almost as heavy as a heavy barrel.
- Neither very accurate, nor especially heat resistant, nor lightweight.
- Only major advantage is cost: this profile is often cheapest.

GAS LENGTH

Carbine length
- 7"
- Harsher action
- More stress on bolt
- More felt recoil
- Only advantage is weight and length compared to rifle length gas system.

Mid Length
- Less stress, recoil, and not as harsh on bolt as carbine length.
- Still gives short handguard length compared to rifle.

Rifle Length
- 12"
- Softest recoil
- Most reliable
- Less stress on bolt
- Gives longer handguard length

COMPOSITION

Chrome-moly
- Generally the most accurate
- Generally the most sensitive to heat (shifted POI when barrel is warm)

Stainless Steel
- Much less sensitive to heat than chrome-moly
- Farm more accurate than chrome, but slightly less accurate than chrome-moly

Chrome plated
- Least accurate of the three
- Most heat resistant


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