Piston wall TNT cobblestone farm

Cobblestone generators on two mirrored sides push walls of fresh cobblestone toward the middle, where a TNT duper drops lit TNT onto an obsidian block every cycle. The TNT goes off while the pistons are still moving the blocks, so they break into items, and the drops land in water on an ice floor. One TNT block is enough, the duper copies it. It suits a singleplayer world or an unmodified server that needs a lot of cobblestone. The repeater delays have to match the video: when the cobblestone outruns the TNT, it reaches the middle and the TNT blows up the duper.

• Cobblestone reaching the middle, with TNT landing on top of it, means a repeater delay is off: check every repeater against the video. The creator's reply, over a dozen builders hit it.
• Plan the collection yourself: hoppers cannot keep up with this output. Builders use water streams or many hopper lines into separate chests. Several builders report it.
• Where duping is banned, the creator suggests a dispenser instead. Re-time it so the TNT goes off while the walls move, three builders who did not saw the cobblestone outrun the TNT.
• On Spigot and laggy servers the delays drift, the duper's minecart derails and the TNT wrecks the farm. The creator advises against Spigot, several builders report it.
• From 1.13 the duper in the video stops duping and the delays change: builders swap in a newer duper, and the top comment (536 likes) gives new repeater timings.