Have a question I have never saw asked, and I have never thought about.
If a certain brand ammo, same 223 or 5.56, different bullet type, but same bullet weight, same muzzle velocity +- a few feet, would it be safe to assume they use the same powder, or close to the same powder burn rate? And that the 10-15 FPS difference was from the bullet type, such as a FMJ vs bonded/HP?
IF.... this may be correct, would it also be safe to assume they would basically give the same muzzle ballistics in different barrel lengths?

no.

1) No.
2) No.
3) No.
4) No.

The more you no.