Do all pellets leave the barrel at the same speed? Absolutely not!
 If the barrel has any choke at all it is impossible for the pellets to all be going the same speed after hitting the forcing cone in the choke, the choke will absolutely accelerate the pellets in the center mass of the shot column, the harder the angle in the choke forcing cone the more acceleration you will have.

 Note: The more acceleration from tighter choke constriction that you can see or measure, the longer the shotstring becomes that you can not see.

 Can you prove this? Yes!
 People do it all the time they just don't understand it, if you have a chronograph a box of shotshells and some choke tubes of different choke constriction just chronograph 5 shells in each choke tube. This will show you how much acceleration is added by choke.

 To get a true reading of velocity of a shotshell you must not use any choke constriction at all.

 What is the tear drop shape in a shot column?
 That is physics displaying itself as good as it gets, that tear drop shape is a cone that was shaped by the forcing cone upon impact in the barrel. Yeah but its a tear drop shape not a cone. Remember in physics for every action there is an equal opposite reaction, take 2 cones and butt them up together, looks like a tear drop to me.

 Ok then, I have seen actual stop action photos of shot strings some very old ones and some newer ones and I was wondering why it appears that there is a wall of pellets or a large clump at the rear of the shotstring and these vary.

 Easily explained: Remembering physics again, here we have another scientific fact coupled with variables, in the first part of explaining shotstrings we noted that the choke will accelerate pellets, well using physics we decelerate pellets as well, the decelerated pellets by force travel back into the shot string that is still in transition in the choke, now the variable, the variable being the wad sealing the gases trying to contain all the pellets as it tries to accelerate from the expanding gases it will start catching up to some of the pellets and collect them back into the wad's shot cup and depending on whether the whole shot column was accelerating from a short barrel or decelerating from a long barrel, it will gather some and that is the clump of shot you see in the photos, the shorter accelerating barrel scenerio will give you a much shorter shotstring everytime.

 What about the draft factor?
 If you mean one pellet following another then passing it, it is almost impossible to apply such a factor.

 Why?
 For the simple reason the shot in a shotstring is always moving away from each other and never being behind one another in a direct path.

 Can you prove that?
 This is simple to prove, when you are patterning a shotgun are you not counting all the holes where all the pellets don't stay behind one another.

 I still don't get it, you know the thing with accelerating and decelerating pellets in a shotstring can you show me?

 Ok, get 4 marbles and make them all touch, take 2 that are to the side and grab them, seperate them slightly away from each other still keeping all in contact, now push together the 2 you have a hold of, note the marble facing away from you goes forward and the one closest to you moves toward you, now imagine this same situation traveling at 1,200 feet per second and happening many times inside a choked barrel. Now you should have a pretty good idea of what causes shotstringing.

 What about various shot metals?
 Number 4 Hevi-Shot, shotstrings between 11 and 12 feet long at 40 yards.
 Number 4 lead-shot, shotstrings between 11 and 12 feet long at 40 yards.
 Number 4 steel-shot, shotstrings between 8 and 9 feet long at 40 yards.

 What about choke tubes?
 Full choke tube, long shotstring.
 Improved Cylinder choke tube, short shotstring.