About
Chunk Sniper is a custom item made especially for Blackblock.
Season 7 note: the Chunk Sniper is not yet available in Gypsum.
Explanation
The chunk sniper allows you to carve out areas beyond the existing world border.
Shift + right click while holding to load chunk sniper charges into the chunk sniper. Each charge allows you to carve one chunk out beyond the world border.
When the world border expands, your carved areas will not be "pushed out" beyond the new border - they will simply be overwritten.