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 ?
I play mostly RTS and RPG. I think g402 is a great mouse but on a couple reviews on youtube in user comments some stated that sniper button is too easy to press accidentally. I don`t think it is a good idea to risk that, better to increase my budget or find some other mouse in that price range.
I don't know about that specific mouse, but in other Logitech mice you can reassign that button to any command you want with the Logitech software. If it becomes a problem you should be able to just reassign it to a not do anything.
I read that many users g502 has problems with mouse double clickingI bought before Asus gladius but I didn`t find it comfortumbly, rubber seem very slippery so I returned it. Does anyone here has Logitech G403? Would shape of Asus gladius be to similar to g403, same grip?