The best direct-fire anti-sniper bullet weapon in the game is another sniper. (Not a section with a sniper rifle in slot 4

). Snipers enjoy
no special advantages vs another specialist sniper, bar small size making initial hitting a tad less than a full sized section.
Next best is an MG unit class weapon, esp if firing from beyond the snipers own rifle range. MG unit classes have a few less problems with the snipers size, and if they miss, usually it is to the adjacent hex, so the splash will cause some "S" anyway (and share the joy with any neighbours he has, too!

).
Andy