On a bb gun what does hopp do?

Question

On a bb gun what does hopp do?

Answer 1

You may mean hop-up system...

This gives the bb a certain degree of back spin.  Thus giving the said bb more (longer) range..

Answer 2

First of all it's called hop-up.

Right, as your bb travels along the barrel, it goes under a little rubber bumb which applies a bit of backspin to the bb. It slows the pellet down minutely but it gives so much more accuracy and range.

It works according to the bernoullie principle which is complicated and which wikipedia could better explain ; )

Hope this helps : D