There are 2 sets of data for the 357 magnum on BBTI.
The first set is from a T/C Encore barrel, cut in 1" decrements. The second set is from a couple of rifles, a few revolvers, and a derringer. The shortest revolver has a 4" barrel.
From the BBTI site:
The 2" barrel used for the Encore 357 magnum testing would have had, at the most, about 0.71" of rifled barrel.
The first set is from a T/C Encore barrel, cut in 1" decrements. The second set is from a couple of rifles, a few revolvers, and a derringer. The shortest revolver has a 4" barrel.
From the BBTI site:
in every case with the T/C Encore the length of the barrel was measured from the end of the barrel back to the breech face
The 2" barrel used for the Encore 357 magnum testing would have had, at the most, about 0.71" of rifled barrel.