Short answer: for most AR-15 rifles, 16 inches is the default: it is the shortest barrel that avoids short-barreled rifle rules with a stock, and it keeps most of the velocity. In Lucky Gunner's test, M855 ran 2,653 fps from 10.5 inches, 2,990 from 16 and 3,110 from 22. Go shorter (10.5 to 14.5) for a compact rifle, and longer (18 to 20) for distance. Past 18 inches the gain drops to about 11 fps per inch.
What each inch is worth
Lucky Gunner tested 16 loads through six barrels and averaged five shots per combination at the muzzle. Averaged across loads, each added inch gave about 107 fps from 7 to 10.5 inches, 65 fps from 10.5 to 14.5, 44 fps from 14.5 to 16, 35 fps from 16 to 18 and 11 fps from 18 to 22. Your barrel will differ a little. For more, see the 5.56 ballistics chart.
| Load (fps) | 10.5" | 14.5" | 16" | 18" | 22" |
|---|---|---|---|---|---|
| Magtech 55 gr M193 | 2,760 | 3,043 | 3,086 | 3,183 | 3,202 |
| Winchester 62 gr M855 | 2,653 | 2,920 | 2,990 | 3,058 | 3,110 |
| Speer 75 gr Gold Dot | 2,275 | 2,502 | 2,550 | 2,614 | 2,649 |
Each length, and what it costs
| Length | Good for | Typical gas | Listings | Median |
|---|---|---|---|---|
| 10.5" | Most compact; SBR or pistol | Carbine | 13 | $189 |
| 11.5" | Compact, a little more velocity | Carbine | 13 | $310 |
| 14.5" | Compact carbine; SBR unless the muzzle device is permanent | Carbine or mid | 44 | $247 |
| 16" | General purpose rifle | Carbine or mid | 89 | $239 |
| 18" | Distance, designated marksman builds | Mid or rifle | 31 | $278 |
| 20" | Full-length rifle, target | Rifle | 22 | $303 |
10.5 and 11.5 inches
These are the short options. You give up velocity (Lucky Gunner measured 337 fps less for M855 at 10.5 inches than at 16), and muzzle blast rises, but the rifle is much handier. An 11.5 was not in the test; it should fall between 10.5 and 14.5. Browse 10.5 inch barrels.
14.5 inches
A 14.5 inch barrel gives up only about 70 fps on M855 against a 16. With a stock it is an SBR unless a permanently attached muzzle device brings it to 16 inches. Have that done properly and confirm the length before you add a stock.
16 inches
The most common length in our catalog, with 89 in-stock 5.56 and .223 listings, and the easiest to build. See the best AR-15 barrels.
18 and 20 inches
An 18 gained 68 fps on M855 over a 16 in the test, and it suits distance builds like the Mk 12. Past 18, the returns are small, so a 20 is mainly about a longer sight radius, a rifle-length gas system or a retro look.
Barrel length and gas system
Match the gas system to the length: see our gas system length chart and handguard length guide. Your twist rate matters too: see profiles and twist rates.
The 16-inch rule
Checked 30 October 2026; this is general information, not legal advice. Federal law treats a rifle with a barrel under 16 inches as a short-barreled rifle, and those rules changed in 2026: see short-barreled rifles explained. Your state may be stricter. With no stock, a shorter barrel can be built as a pistol: see how to build an AR pistol.
One barrel at each length, with today's prices
Lowest price across the stores we track:
Faxon 5.56 10.5" Gov't/SOCOM contour
Pros
- 1:8 twist, nitride
Cons
- Check gas length on the listing
10.5 inch
Faxon 11.5" Big Gunner 5.56
Pros
- Mid-length gas
Cons
- Heavier profile
11.5 inch
Ballistic Advantage Classic 14.5" M4
Pros
- 1:7 twist, chrome lined, threaded
Cons
- Needs a permanent device to reach 16"
14.5 inch
Faxon Match 223 Wylde 16" Gunner
Pros
- 1:8 twist, nitride, 223 Wylde
Cons
- Match contour, not pencil
16 inch
Ballistic Advantage Modern 18" rifle
Pros
- Rifle-length gas, .223/5.56
Cons
- Long handguard to suit
18 inch
Faxon Duty 5.56 20" Gunner
Pros
- 1:8 twist, black nitride
Cons
- Long rifle overall
20 inch
Compare a full rifle at any length in the AR-15 builder.
Frequently asked questions
What is the best AR-15 barrel length?
For most rifles, 16 inches. Choose shorter for compactness and longer for distance.
How much velocity do you lose with a 10.5 inch barrel?
In Lucky Gunner's test, 337 fps with M855 compared with a 16 inch barrel: 2,653 against 2,990 fps.
Is an 11.5 better than a 10.5?
It should be a little faster, since each inch in that range averaged about 65 fps in the test, though 11.5 itself was not tested.
Is an 18 inch barrel worth it over a 16?
If you shoot at distance. In the test it added 68 fps with M855. It also adds length and weight.
Is a 14.5 inch barrel legal?
With a stock it is a short-barreled rifle unless a permanently attached muzzle device brings it to 16 inches. Check our SBR guide and your state law.
