i can answer that for you, despite not owning a 402.
The "sniper" button is harder to press than the other buttons. So, no.
what that button does is change the DPI on the fly. lowers your sensitivity for smaller movements, then reverts back to normal sensitivity for run n gun.
You can get *very* detailed reviews (including the buttons and shell) of just about every major gaming mouse by watching Rocketjumpninja's videos:
https://www.youtube.com/channel/UCGJaDZC7PChgd-XMwcbZkiw
you don't really need a DPI-changing button. You should be able to set up your mouse sensitivity well enough so that you can snipe, snap and track all with the same setting. at most, you will want a zoom, which any respectable FPS game has
(EXCEPT REFLEX! *headdesk* WHY_U_NO ZOOM, NEWBORN!!).
What game is it that you play ?