Straight-walled cartridges, such as the .450 Bushmaster and 350 Legend, have sides that are the same diameter from front to back, as the name implies. When looking at the cartridge from the side, you'll notice a straight line running from the rim to the end of the case, narrowing only at the bullet.
A bottlenecked cartridge, on the other hand, will taper down because the case diameter is larger than the diameter of the bullet. The "shoulder" is the most visible feature of a bottlenecked cartridge.