Agreed Jim,
After seeing how the carbine round fits on the OP- JB's barrel below:
Using the Carbine rd you'd think it would be about worn out.
Look Here at some pictures shared by another member from the CMPforum.
Showing a carbine rd vs a m2 round.
The Carbine barrel below measures a hair over 1.5
M1Carbine Round:
M2 Round:
A Thank You to Member Orlando for the picture comparison....
A Good Weekend To All....