Tube end on left in picture is a Double Flare. Tube end on right in picture is a Bubble Flare. With a flaring tool in had you can form these shapes in a minute. Ilustrations below show the fixed die and collet holding the pipe in green, the punch is in purple, and the pipe is black. One stroke of the piloted flaring punch will form the bubble flare. Note that the piloted punch will bottom out on the die to control height of the bubble. If the tube is not positioned high enough the bubble will be too small. A second stroke using the conical punch will push ths bubble down to form the Double Flare.

Bubble flare forming shown above. Double flare forming shown at right. The tube nut shown below is similar for both forms, but the seat in the mating part will differ in shape to match the style of flare.