not all guns have 55 degrees

 

^yep

In real life, it would be 45 (http://www.physics.rutgers.edu/~zapolsky/381/aim.pdf)
However, in NF, it's 55, but with many guns that have the ability to go the the optimal 55 degree angle, I sometimes prefer to shoot at a 40-45 angle. This is because the range difference is usually negligible (especially for those guns that can reach such an angle) and the lower the angle, the less travel time your shells have. For this reason, I find it better to use higher angle shots when kiting, and lower angle shots when rushing for line fighting (Sodak vs NorCar for example).

For larger guns, yes, but not for ALL guns.
http://trainworld.us/NF2/Gun_Data/index.html