Exactly, this bullet has to travel through the water from the start (there is probably water in the barrel) which would prevent it from reaching its top speed at all.

The gun looks like a Glock 17, which is about 20cm long, the muzzle bloom looks about the same length and the bullet travels twice that distance before loosing forward momentum, so about 60cm or around 2ft - more than the .50 cal, but not 8ft.