USA: Early M4 hulls with L40 gun?

[jp10]

From reading the manual, and looking at the diagrams it seems that only the threaded front part of the breech where the barrel screws into would be in the measurement.
3. DESCRIPTION OP GROUPS.—a. Gun group.—(1) Tube.— The tube (fig. 1) is formed of one piece of alloy steel. The rear end of the bore is suitably tapered to form the chamber, and from chamber to muzzle the bore is rifled with a uni form right hand twist of one turn per 25.59 calibers. The exterior of the breech end is threaded to screw into the 75-MM TANK GUN M2 breech ring, and the shoulder immediately forward of the threads contains a keyseat for the breech ring key. Forward from the breech ring the exterior is cylindrical and smoothly finished for bearing in the mount for approximately half the length of the tube. The breech face of the tube is re cessed on each side of the bore to form extractor camming surfaces (fig 3).

https://www.ibiblio.org/hyperwar/USA...Fs/FM23-95.pdf

if you want to see the diagrams.