The drop at comb and heel is a fixed set of dims= laying a straight edge from the breech where the barrels close to the receiver, and thus back towards the stock's butt-- barrel length affects the pitch of the gun- usually down pitch from the muzzles.
Old school measurement for pitch- place the gun with the barrel/barrels flush to a 90% doorjamb- with the butt flat to the floor- the stand-off distance from the top rib at the muzzles to the edge of the jamb is the pitch as measured. Shape of the butt plate or recoil pad can also affect the pitch.
Many competition clays shooters prefer an almost zero pitch, whereas small game (rabbits , etc.) upland hunters will do better with a shotgun with an amount of down pitch.
This is my amateur description- best one I have ever read was Captain Paul A. Curtis's book 1934-Penn Publishing-- "Guns and Gunning"--
I have a 12 gauge L.C. Smith with two factory sets of barrels- 30" and also 28"--I have shot it a great deal over 40 years, and it fits me like a pinkie ring on a Goombah-- same stock, pad, receiver, LOP to pad from front trigger, same DAC and DAH, but measuring the down pitch using the above described "door jamb method", I get 2&1/2" downpitch for the 28" barrel set, and 3&1/8" downpitch with the 30" set of barrels in place- The DAH and DAC stays exactly the same, regardless of the barrel length.
