This interests me - because I have always 'perceived' my guns as being different lengths, whereas measurement indicates the are (almost) the same. All my guns (I have over a dozen in the cabinet) are between 14 3/8 and 14 5/8 when measured on a gauge. One feels noticeably short, a Darne, and the reason is that the standing breech is right above the triggers, whereas on an English gun it is a little forward. This, with the Darne, whilst the trigger is in the right place with respect to the shoulder, the standing breech is a little nearer the eye.
Similarly, single trigger guns 'feel' longer - because the measurement made to the trigger is about where the rear trigger on a double trigger gun would be.
I also have a theory that on our side of the Atlantic we tend to have stocks a little longer, a little straighter in drop and with more cast. The reason is we shoot with the gun mounted a little more 'obliquely' rather than fully 'square on' as we are generally taught (for a right hander) to have the left foot rather forward and to bring the gun well up to the face, raising the shoulder slightly as we do so.
The topic is discussed in detail in John Brindle's Shotgun Shooting: Techniques and Technology.
