...energy is just a calculated number. Meaning very little by itself...
This is simply not true.
Kinetic energy is a physical quantity which has been experimentally proven to be the potential of a moving object to do work. In the context of ballistics, it relates to potential of a projectile to cause damage/penetrate.
Energy = force, but without considering how that force is applied, its just a number.
Energy does not equal force, but force can be calculated from the change in energy and the distance over which the energy was changed.
Force = (Energy change) / (distance over which the energy change was accomplished)
In other words, if the energy of a projectile is reduced by 100ftlbs as it travels through a 1 foot thick target, the total amount of force applied to the target is 100lbs. It should be noted that not all of this force is applied in the direction that the projectile moves.
It is useful to understand energy, and it is certainly useful for more than just comparisons. However, it does not tell the entire story of terminal ballistics by itself. In fact, it is only a small part of the overall, very complicated, picture.
Velocity = energy and energy relates to "killing power".
Velocity is not equal to energy although kinetic energy can be calculated from velocity (squared) and mass.
Neither velocity nor kinetic energy can be directly related to "killing power" in general, although both of them certainly have an impact (pun intended) on the potential of a projectile to cause damage.
...let me tell you that the lbs/ft of energy calculation gives a fictitious energy unit and only supports the argument for high velocity/low mass bullets.
This is pure bunk and contradicts centuries of established science. Kinetic energy is not fictitious in the least and anyone who wants to know the truth doesn't have to work too hard to find it given the huge amount of information that is readily available to everyone with access to the internet.
Kinetic energy was not developed to sell anything, it was discovered (it already existed before scientists quantified it) long before anyone was arguing about velocity and mass of bullets. If you believe otherwise, then please provide some evidence of which ammunition company Gottfried Leibniz was working for when he discovered kinetic energy in the late 1600s? Also, which bullet was Emilie du Chatelet marketing when she did her experiments confirming the validity of Liebniz's energy theory in the early 1700s? What where the Bernoulli brothers and Gravesande selling?
It's one thing for someone to admit they don't understand what kinetic energy is, or how it affects the real world, or how it relates to terminal ballistics, it's another thing entirely to claim it is just a fictitious number, that it was developed for marketing ammunition or to support a pet theory about ballistics, etc.
For around 300 years, the scientific/technical community has accepted the equation as the correct quantification of kinetic energy and has not only experimentally verified it repeatedly, but also relied upon it to produce accurate and precise results in the real world.
Kinetic energy is not just the product of an equation or solely a mathematical construct, it really does exist in the real world. It really does affect the real world. It really does provide a measure of the potential of a moving object to do work. Understanding it really does allow a person to predict certain aspects of how a real-world moving object will interact with other real-world objects. It’s certainly not the entire story, when it comes to terminal ballistics, but it is definitely part of the story (a small part) and understanding it properly will provide insight that is obscured if one tries to ignore it.
There's a lot of readily available information out there for people who want to educate themselves on the topic.
http://hyperphysics.phy-astr.gsu.edu/hbase/ke.html
https://en.wikipedia.org/wiki/Kinetic_energy
http://www.auburn.edu/academic/classes/scmh/1010/Kinetic Energy.php
https://www.khanacademy.org/science...gy/kinetic-energy-ap/a/what-is-kinetic-energy
http://thesimonscenter.org/wp-content/uploads/2017/05/IAJ-8-2-2017-pg62-68.pdf
As far as the OP's question is concerned, the difference in velocity due to the barrel length difference is not likely to have any significant effect on the lethality of projectile. Not because velocity, or energy, or momentum decreases are meaningless or made up or fictitious, but because the velocity/energy/momentum decrease is not significant when going from a 20" barrel to a 16" barrel.