why do some ar-15 barrels have that reduced diameter section?

[OC_]

You know what im talking about? Some barrels have a section of the barrel thats been turned down so its a little smaller than the rest of the barrel. Its usually after the front sight.

 

[HorseSoldier]

It's for mounting an M203 grenade launcher.

 

[BsChoy]

Thats an launcher cut...it's so the M203 grenade launcher can be attached

 

[Hanzerik]

I want to say it's for grenade launcher mounting (M203)

ETA: beat by a minute

 

[Jim Watson]

Of course it is rather difficult for an American Commoner to get a grenade launcher to clamp on there. But it is still way kewl and tackytickle. Lets people know you are a no hu-hu Operator.

 

[HorseSoldier]

Yeah, it serves no real purpose unless the goal is just to have a high fidelity M4gery.

 

[GCBurner]

It's so you can say your rifle barrel is "Mil-spec."

 

[greyling22]

up at the front it's because of the grenade launcher. under the handguards it's to reduce weight.

 

[shep854]

It goes back to when the M16A2 was being developed. The Marine Corps wanted a heavier barrel, but then (as mentioned above) the M203 grenade launcher would not attach. Someone figured that retaining the original barrel thickness behind the front sight assemble would solve this problem, as well as shaving some weight off.

Using the same profile barrel on civilian rifles is simply common sense, to hold down costs. There are heavy barrel options available, for those who want them.

 

[Birddog1911]

It's so you can say your rifle barrel is "Mil-spec."

There are plenty of barrels out there with the launcher cut that are far from mil-spec.

 

[Tirod]

The launcher cut is now dropped from the spec for the present, and future contracts. Like quad rails, the forward assist, and the disposable magazine, it was an institutional requirement, not a superior enhancement of the design. The cuts are now proven to be a weak area and cause early failure in back to back mag dumps.

Moot point in a semi auto gun; for military use, no longer wanted on a carbine. The NEW grenade launcher can be used with it's own stock and chest holster. The cuts are now curio and relic collectable fodder, like carry handle uppers. Nice period dress up for an aging retro Barbie.