Velocity vs. Barrel Length.
During testing with my 32 inch Winchester 37 I have noticed that I consistently get velocities higher than published velocities both for reloads and factory loads. There can be many reasons but three obvious possibilities come to mind. Either it's due to the barrel length, the tight choke or a chronograph that has a positive bias. Could be the chronograph as it is a bargain priced unit but lets use this opportunity to examine several truisms and at the same time narrow down the actual reason for the increased velocity.
Myth - The velocity of a shell fired from a long barreled gun will be greater than the same shell fired from a short barreled gun.
This myth has also been around forever with little or no proof. It may very well be true when comparing a pistol with a 5-inch barrel to a rifle with a 24-inch barrel but my concern is shot shells and target shotguns. This should be an easy thing to test so first I'll test to see if it is true. Then if it is true I'll try to see how important the relationship really is. Here I would also hope to determine if speed variation caused by barrel length is important enough to consider when choosing a load or barrel length.
1. Shoot and collect patterns and velocities from 2 barrels of different length.
2. Evaluate and calculate 70 % circle diameters - use these in other studies.
3. Run simple statistics on tabulated results to test hypothesis
4. Try to get a statistically significant relationship between velocity and barrel length.
5. Draw conclusions.
Technical precautions and procedures:
1. Eliminate as many variables as possible.
2. Gun and barrel differences and other thoughts.
Running a few simple statistics against the data we see that the velocity for the 24-inch barreled gun has an average velocity of 1200.0 for the ten shots with a standard deviation (SD) of 10.4. The 32-inch barrel has an average velocity of 1207.6 with a standard deviation (SD) of 22.
VARIABLE MEAN VARIANCE STD DEV 1 1200.0 107.5556 10.3709 2 1207.6 482.7111 21.97069
What this means:
Variable 1 is the 24-inch barrel and variable 2 is the 32-inch barrel. Statistically, we can be 95% certain the "real" average velocity (for this shell fired from this gun) is within 2 SD of the average found by our ten shot average.
We can say the "real" average for 24-inch is between 1200-20.8 and 1200+20.8 or between 1179 and 1221. The "real" average for 32-inch is between 1208-44 and 1208+44 or between 1162 and 1252. These two ranges overlap and as a matter of fact the 24-inch is completely within the range of the 32-inch. Statistically, based on the results from this test they are not different.
Intuitively this also makes sense. The averages are only 7 f/s apart while the barrel length difference is huge. If there was an impact on velocity because of barrel length it should have shown up with an 8-inch barrel difference. There is a slight bias with the short barrel having a 7 f/s lower average velocity but that equates to less than 1 f/s per inch of barrel length. That is hardly anything to worry about even if true.
Barrel length has NO or minimal impact on velocity as per this test. I found no statistical evidence to support the myth. Review again the test parameters and gun variations. This finding only holds for the above-mentioned shot shells, range of barrel lengths tested, and the .690 choke -- but they are consistent with most Trap shooting configurations.
Real eye opener:
I got more consistent velocities from the 24-inch barrel than the 32 as seen in the Standard Deviation. The 24-inch also gave more consistent and tighter patterns but that might be explained by the Winchoke tube choke as described above. This will require more research to find out if it is simply an anomaly or something to think about.