My C9 3.5" barrel does not get 400 ft/lbs Energy from a 124 NATO round......
but does reach 382 ft/lbs.

This makes me think that a 4" or longer might be used for NATO spec's
that is printed on the ammo boxes.

Even my 5" has trouble getting the printed spec's that come with some ammo.
Must have been from that 7.5" pistol.......