Question: Does a shotgun recoil before the shot leaves the barrel & what effect does barrel length & choke have on recoil?

Answer: First let me say I am neither a ballistician or an engineer but have been shooting & studying shotguns for over 50yrs so will try to give as educated an answer as possible. Most of my data is, same as me, somewhat aged but based on principles which have not changed. Up front let's state there are two types of recoil, the actual mechanical recoil & perceived recoil or what is often termed "kick". We will be discussing the actual which can be measured. what is usually referred to is known as free recoil or how much the gun would recoil in a free state without any additional resistence. This is based on the law of motion which states that every action has & "equal reaction". The free recoil of a firearm is composed of three elements:
1; The reaction of accelerating the projectile from a state of rest to the velocity it obtains upon reaching the muzzle.
2; The reaction of the acceleration of the powder charge, in the form of gas, from it's position of rest to the C/G of the bore & chamber. EG the part of the gases immediatly behind the projectile will have obtained muzzle velocity while that in the chamber will not have moved at all sothe c/g of the bore & chamber will be the median of the entire charge.
3; The reaction of the muzzle blast as the projectile leaves the bbl thus realeasing the gas which rushes out & pushes the gun rearward just as a rocket engine would.
Note #'s 1 & 2 begin immediately upon ignition of the powder charge & end, except for the inertia imparted, upon the projectile leaving the muzzle. As long as total ejecta weight, weight of powder charge & muzzle velocity are known these two elements can be calculated with extreme accuracy. The 3rd element is based on area of muzzle, muzzle presure & mass rate of discharge which except for the area are mostly unknown elements not readily calculated.
However some reasonably simple formula have been worked out which give fairly close approximations of the recoil. One formula for shotguns is recoil velocity of gun = [projectile wt (shot & wads) + 1.25 to 1. 5 x wt of powder charge] x muzzle vel / 7000 x wt of gun. The 1.25-1.5 times powder charge will vary depending on lgth of bbl, type of powder etc. For long bbl with fast powder use 1.25 for shorter bbl with progressive powder use 1.5 & vary in between for various combinations. After obtaining recoil vel then ft lbs of recoil energy can be obtained by the kinetic energy formula of 1/2 MV^2. Element 1 produces the major amount of the recoil & is the one which operates recoil operated auto's whether blow back or short or long recoil locked breech type. Indeed if recoil came only from element 3 then a blowback type as in .22 auto's & many pocket pistols in .25, .32 &.380 auto with bbls fixed to frame with only breech block moving would not operate. Element 3 is normally the least & is the part on which "ports, recoil brakes etc work. This should I believe give ample evidence for the first part.
For the next part tests were made many yrs ago in England by W D Borland using a pair of pendulumguns. One was the standard Webley Field Pendulum Gun of the day while the other was known as the Dartford Pendulum Gun. The Webley had a fixed bbl of 30" while the Dartford had a 22" bbl to which extensions could be fitted giving lengths from 25" to 40". Both were chambered for the 2 1/2" 12ga shell of the standard British game gun of the day with normal forcing cone of approx 3/8" length. The bbl & all extensions of the Dartford had a .729 bore an with the exception of one 8" extension were cyl bore. The one was full choked to a dia of .689 or 40 points of choke. The Webley had a taper from forcing cone beginning at point of .735 dia to .733 dia at 12" from breech face. This .733 dia was maintained to the beginning of choke which was fully choked to a dia of .690 or 43 points of choke. Tests were made of 8 different loads all in the standard length hull of the day. Seven of these were with the bulk powders common for the day. Four were the standard load with a 3 dram charge & 1 1/16oz of shot. One was a "Hi-Velocity" load of 3 1/4dram 1oz shot, One a "Low Velocity" load of 2 3/4 dram with 1 1/4oz & a "Very Low Velocity" load of 2 1/2drams & 1 3/8oz of shot. The last test was a standard load with dense powder "Improved Ballistite" in a 3 dram equiv load with 1 1/16oz of shot. Several interesting points were brought out in these tests. They are too long to go into in detail but a summary was provided which we will look at. Results are given for velocity as the average over 20 yds & for recoil as the velocity of a 6 1/2 lb gun. Rather than actuals we will be looking at differences. Differences in muzzle vel would be approx twice that of the 20yd average.
1st (Dartford gun) average decrease in vel from a 30" cyl to a 25" cyl was 26fs; recoil was reduced by 0.29fs (note this is much less than would be expected for a 52fs difference in MV).
2nd(Dartford gun) Average increase in velocity of 30" f choke over 30" cyl 22fs; average "decreaase" in recoil 0.12fs.
3rd (both guns) Average increase in vel of Dartford gun over Webley gun 35fs (both 30" f choke bbls); average increase in recoil of D gun over W gun 0.87fs (this is proportionate to vel varition).
Some explanations:
Velocity loss in shorter bbl obvious: Less time for gasses to push. Unproportional recoil: as bbl was shortened lowering vel, recoil would be expected to drop, however shorter bbl resulted in increased muzzle pressure thus increasing element 3 of recoil thus nearly neutralizing the efect of the vel loss.
Choke vrs cyl. the shot charge hitting choke cone would be expected to check vel thus decreasing muzzle vel & this slight check would also check recoil to a slight extent. I believe this is what happened. Remember vel was recorded over 20yds. The tighter mass of the shot leaving the muzzle resulted in less air drag on individual pellets for a short distance thus showing the increas in ave vel. Actual muzzle vel was probably proportional to the decrease in recoil.
Best for last; Now I know .004 is only a very slight back bore & the slight taper of .735-.733 to the 12" point can not really be called a "Long Forcing Cone" but it is interesting that the gun with min bbl dimensions had in every load tested both higher vel & higher recoil. The two loads having the least difference were standard loads of "Smokeless Diamond" powder, the only difference being one used brown felt & the other white felt as filler wads. They varried by +4fs & +7fs respectfully from the same load in the Webley gun. The load giving the most variation was the std "Ballistite" load @ +64fs in the Dartford gun. Will leave you to your own conclusions on this one as I do not have enough up to date proof with modern powders & wads to burst anyones bubbles. Will only say there are many existing claims of "Increased Vel with Lowered Recoil" that I am some what skeptical of.
Anyone who has "Good Data" that either correlates or contradicts this post it to the bbs or add another listing here or I will work with you & Dave & we will update this post. I am not trying to prove what I know, only to study & learn. Resource for this was "Hatcher's Notebook" by Julian S Hatcher, Major General, USA, Retired & "The Modern Shotgun" by Major Sir Gerald Burrard. Have had my copies for many yrs but believe both are still in print.

Submitted by: M M Fulks
Dave Weber
DoubleGun Evangelist
In Charge But Not Responsible