6x47,
Most barrel makers list the M24/M40 contours as being the same. There may be an exception but Bartlein and Rock Creek both state they are the same.
Contour choice is usually based on weight and balance vs use intended. In my situation, building and shooting tactical competition rifles, the medium Palma or Rem/Varmint with a longer tenon are the usual choices. The M24/M40 tends to be just a tad too much weight by the end of a long day of competition and don't offer any significant improvement over the Palma or Varmint contours. But opinions vary so I can only vouch for the competitions and competitors I know.
Regards.