In my experience, the 1896/1898 rifle barrel-band and 1899 carbine barrel-band fit the same forearm dimensions and barrel contour. Both types of bands are positioned the same distance from the front of the receiver.

IMHO - Not a lot of point in measuring, since the carbine one is solid and retained by a band-spring.
The rifle barrel-band is adjustable (to a degree) and retained by a Stock 'cross-pin', once in position and 'clamped'.

Both types of barrel-bands are slightly tapered on the inside, hence the need to orient them with the "U" on the Right-Side of the Arm.

BTW - The model 1896 carbine (sight-protecting) barrel-band has different interior dimensions, since it is positioned closer to the receiver, where barrel diameter is greater