M1903 vs 03-A3 Barrel dimensions

[Merc]

How did the dimensions of the 03-A3 barrel compare to the M1903 barrel? Were they identical? Interchangeable?

[mhb]

are functionally interchangeable - that is, the threaded shank dimensions are the same, and either can be installed on any 1903 or A3 receiver.
The external dimensions are the same with the exception of the shank diameter: the 1903 shank is smaller because the rear sight collar must be installed on it, while the A3 shank is left larger. The A3 shank can be machined to accept the 1903 rear sight collar, if necessary, but it is a job for an experienced gunsmith, since the collar needs to be correctly indexed on the barrel by means of a crosspin and longitudinal pin.

mhb - MIke