As Issued 1944 IBM M1 Carbine

[majspud]

Sorry, thought you were referring to the "I" cut under the front sight, which all IBM barrels have and is visible on some even though the front sight is installed properly. Some IBM barrels have an additional cut aft the "I" cut.

https://www.milsurps.com/images/impo...Barrels1-1.jpg

Curious about the plain versus grooved barrel. As mine is very late production, is it a deleted milling process, early vs. late? Or are the little 'ribs' to aid in heat transfer to cool the barrel more quickly?

T