Why the OD groove?

[Ownerus]

in the M4 type AR barrel?
My military experience with the M16 dates back to the M16A1 so the M4 and it's accessories are new to me. Looking at the vast assortment of AR barrels I see this OD groove 4 or 5 inches back from the muzzle on barrels that are M4 type. Can someone enlighten me on what the purpose of this groove is?

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[Kruel J]

Grenade launcher cut.

 

[Ownerus]

under barrel mount?

 

[Kruel J]

Yes sir.

 

[Ownerus]

thank you

 

[Kruel J]

Good info found here:

AR15 Barrels: Profiles, Fluting, and Dimpling (Part 4)

This article is part 4 of a 5-part series about AR 15 barrels. In this article, you will learn more about the different AR 15 barrels profiles and the importance of fluting and dimpling.

www.at3tactical.com

"On an assembled URG (upper receiver group), the M4 barrel is identifiable by the "M4 cut" – and here we're not talking about feed ramps, but rather a narrower "notch" in the barrel, forward of the gas block, used to mount the M203 grenade launcher. While most civilians don't have M203s, the barrels are still used to create "M4gery" rifles modeled on the military weapons."

 

[Duge]

Learned something new here.