>>63919874
>what is the functional difference between 124 and 115gr FMJ?
Energy = mass × velocity. A higher weight bullet of the same velocity as a lower weight bullet will have more energy. Different weights also have different ballistics (velocity, trajectory, energy, speed, range, etc.).
>Why is it even a thing?
Some people want softer shooting, quieter rounds. Some people want barrier penetration. Some people want hotter rounds. Etc.
>Does 124 work better in certain guns or vice versa?
Yes.

>>63919894
Did you shoot any fish, anon?

>>63919899
OP's pussy is admittedly BTFO.