Short answer: the M4 is the US military's carbine: a 14.5-inch barrel and a select-fire trigger group (three-round burst on the M4, full-auto on the M4A1). An AR-15 is the civilian semi-automatic rifle it's based on, usually with a 16-inch barrel. Civilians can't buy a new select-fire M4, but they can build or buy a semi-automatic "M4-style" rifle with almost every other feature.
AR-15 vs M4 side by side
| M4 / M4A1 (military) | Civilian AR-15 / "M4-style" | |
|---|---|---|
| Fire modes | Semi + three-round burst (M4) or full-auto (M4A1) | Semi-automatic only |
| Barrel length | 14.5 inches | 16 inches, or 14.5 with a muzzle device pinned and welded to reach 16 |
| Barrel profile | M4 profile with a cut for a grenade-launcher mount | Any: M4, government, lightweight, heavy |
| Gas system | Carbine length | Carbine or mid-length on 16-inch barrels |
| Feed ramps | Extended M4 feed ramps | Usually M4 feed ramps too |
| Who can own one | Military and law enforcement | Most adults, subject to federal and state law |
Why civilians can't buy a new M4
Burst and full-auto fire make the M4 a machine gun under federal law. The 1986 Firearm Owners Protection Act (the Hughes Amendment) banned the transfer of machine guns made after May 19, 1986 to civilians, so the only select-fire rifles civilians can own are older, registered ones that sell for many thousands of dollars. The M4 entered service in 1994, so there are no civilian-transferable M4s.
The barrel length question
On a civilian rifle, a barrel under 16 inches makes it a short-barreled rifle under the National Firearms Act, which needs ATF approval first. That's why civilian "M4" rifles either use a 16-inch barrel or a 14.5-inch barrel with the muzzle device permanently pinned and welded so the total length reaches 16 inches. Check federal and state rules before you build anything shorter.
Building an M4-style rifle
Most "M4" parts are ordinary AR-15 parts: an M4-profile barrel, a carbine gas system, a collapsible stock on a carbine buffer tube and M4 feed ramps. Start with upper receivers and barrels, or open our budget AR-15 starter build in the AR-15 builder and swap the parts you want.
Frequently asked questions
Is an M4 the same as an AR-15?
No. They share the same design, but the military M4 has a 14.5-inch barrel and burst or full-auto fire, while a civilian AR-15 is semi-automatic with a 16-inch barrel or a pinned and welded 14.5.
Can a civilian own an M4?
Not a new select-fire one: federal law bans transfer of machine guns made after May 1986. Civilians can own semi-automatic M4-style rifles.
What is an M4 feed ramp?
Extended ramps cut into the barrel extension and upper receiver that help rounds feed reliably. Most modern AR-15 uppers and barrels have them.
Is a 14.5-inch barrel legal on an AR-15?
On a rifle, only with ATF approval as a short-barreled rifle, or with a muzzle device permanently pinned and welded so the barrel measures at least 16 inches. Check your state's rules too.
General information, not legal advice. Federal and state laws change; check current ATF guidance and your state's law.
