The only reason for someone to think muzzle energy has anything to do with free recoil energy would be a misguided notion of what "conservation of energy" means.
That's not what I said--I said it was proportional to the reaction force imparted to one's hand, which makes comparisons more convenient since energy numbers are often published and don't have to be calculated.
That's not energy that does that, but force.
Isn't that what I said?
There's an inverse relationship between force, and amount of time over which a momentum impulse is delivered. I.e., for the same momentum, less time equals higher force. Thus, a higher velocity bullet with the same momentum, will exert more recoil force for the exact same recoil impulse, because of the shorter dwell time of the bullet in the barrel (assuming equal barrel lengths).
Now, the only way to get the same momentum with a higher velocity is to use a lighter bullet. And a lighter bullet at high speed will always have more energy for the same momentum.
And that higher level of energy will give you some idea of what the underlying force will be, all else being equal--it was just a simple short-cut, not a full explanation of how energy relates to felt recoil.
However, it's the higher velocity which causes both higher energy and "snappy" recoil. Like raincoats and umbrellas; one does not cause the other, a third factor causes both.
That's valid given equal momentum, but when comparing two close but unequal calibers, such as 9mm and .40 S&W in this case, for which velocity and dwell times are very similar for the common bullet masses of similar sectional density, the relevant difference between the calibers comes out to bullet mass, which of course relates directly to force. What I wonder is, according to the collective subjective perception of people, whether 9mm is ever snappier than .40 S&W. To me, 9mm is lighter but just as snappy, however most others seem to think that .40 S&W is
always snappier. It all comes down to what "snappiness" really is, which may not necessarily correspond to your technically sound definition.
I shoot the quick SNAP of a 357 better than the longer SHOVE of a 45. Why? Cause I am old skool Revolver and the 357 K-Frame is what I first trained with and mastered.
As stated earlier, I prefer "snappy" recoil, too. If there are more than a few who do, however, then they haven't spoken up much about it.
I belive your dad is right. I have meet more than one vet that has told me they did it their self, I have personally never tried it nor would I. But as many people as I have meet that claim to have done in, I believe it was possible for a Vietnam military issue AK to fire the 5.56
While it's certainly possible to chamber the AK in 5.56x45mm, I heard that the very same rifles could fire either that or 7.62x39mm, which seems pretty farfetched to me. I believe the rumors started because more than a few GIs had been wounded or killed by 5.56 rounds, but it was probably by M16s that the enemy picked up. As for some vets claiming to have done it themselves, well, you may also be able to fire a 9mm round in a .45 ACP handgun sometimes, but it doesn't mean that it would be effective and the gun would function correctly (that's a big difference in bullet and bore diameter there).
