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So, is the reason you can get 1600 FPS because of the 125 grain bullet VS. the 158 grain bullet? Or is it the powder? I'm think I'm getting conflicting info here, as the previous post says you can only get those speeds from a rifle, not a handgun.
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It is a bit of both. Consider first that every caliber has a SAAMI maximum design pressure. Now, apply that same pressure to push a 125gr projectile v a 158gr projectile. The lighter projectile will travel faster when pushed by the same equal pressure. You should also note when looking at reloading data that the powder charges are reduced as you increase bullet weight.
Lets complicate this a bit more. A faster burning powder (as notated on the powder burn charts) develops pressure faster than a slower burning powder. If you are seeking the highest velocity for a heavy bullet, you would tend to want a slower burning powder (it takes a bit longer to get that bullet moving, so you want your pressure to rise more slowly to give more push to that bullet as it develops momentum going thru the barrel).
Now, add in the variable of short v longer barrels. A 2" pistol barrel cannot provide anywhere near the velocity of a 6" barrel with the same charge. Why?, because the bullet leaves the barrel before the pressure has enough time to peak and provide maximum push. This is why there are some special loads developed for 2" barrels that cannot be used in longer barrels.
Take the same load you developed for your 6" barrel and shoot it in your 24" rifle barrel. The rifle barrel develops much greater velocity. The pressure developed has a much longer time to push the bullet, thus increasing velocity, before the bullet exits and relieves the pressure.
One last point. Pressure curves are NOT linear across the range of powder charge that is recommended for a specific bullet. In other words, each additional 0.1gr of powder will not always provide the same incremental increase in FPS. This is one of the signs reloaders using chronographs look for when developing loads. If you are regularly seeing a 30fps velocity increase for each 0.1gr increase in powder charge, then start seeing only 10-15fps, you have reached the maximum useful charge for that combination and are riding on the edge. In other words, the additional pressure being developed is not being relieved as quickly as the lower charges. At that point, a wise man would back off a few tenths of a grain.