Keegan said:
most of the time a lighter round for instance 115 grain on 9mm would have a higher veolocity than lets say a 147 grain but i have seen where that same round doesn't have a higher muzzle energy than a 147 grain. Isn't usuall the higher velocity would have the higher energy?
Go back to your first post. Remember where you asked "If everything else is equal"? You can't compare a commercial 115-grain round against a commercial 147-grain round the way you're doing, because most likely everything else is NOT equal. Even if you can find both bullet weights from the same manufacturer and in the same "model" of cartridge -- you have no way of knowing if they both have the same powder charge (they probably don't), or even if they both use the same powder. Bullet weight isn't the only variable in loading ammunition.
Take another look at that article I linked to, from
Shooting Times. It's for .45 ACP rather than 9mm, but the principles are the same. Look at Table 1. Three bullet weights, all loaded to 5.0 grains of powder, yield identical recoil energy. How is that possible? Simple -- for every action, there is an equal and opposite reaction. If the initiating action is the same in all three cases (i.e. the energy released by burning 5.0 grains of gunpowder), then the reaction must also be the same. However, they won't all
feel the same to the shooter. And they won't all have the same muzzle velocity.
But, again, your original question was "if everything else is the same." That means the same gun, the same brass, the same bullet, the same primer, the same powder ...
everything is the same except the velocity. But wait -- if
everything is the same, how can we change the muzzle velocity? The ONLY way to do that with the same bullet and the same powder is to use less powder. Velocity = mass x acceleration. The barrel length is the same, the mass (bullet weight) is the same, so the only way for the same mass to be accelerated to a lower velocity in the same distance is to hit it with less energy. That means using less powder. Less powder ==> less energy ==> less recoil.
Later in that article (Table 3 and Figure 3) they compare the recoil from three different bullet weights all producing the same power factor. But now all things are not equal, because not only are the velocities different, Table 3 shows that it takes different powder charges to get the three different bullets to the same power factor. Surprise, surprise, the one producing the most recoil was the one using the most powder (action <==> reaction). But, although the amount of measured recoil (actually, barrel flip in a Ransom rest) varied, it didn't vary all that much. Note, by the way, that they didn't start the graph at zero -- the lowest number on the Y axis is 30. This distorts the visual representation of the data (which happens to be a pet peeve of mine). If you extend the bottom of the graph all the way to zero, in proportion, the variations across the top won't appear very significant. In fact, the variation from the 185-grain to the 230-grain is 7.8 percent. That's a difference, but it's not huge.