Grindstone

 is a Tier 5 Tank that upgrades from Stacker, Rainmaker at level 60.

Design
The greatly resembles the Stacker but each set of three barrels has a gradient like Rainmaker. It is not one smooth gradient across all 6 barrels, but 2 separate gradients contained to each set of 3 barrels. Each set of barrels has the typical Rainmaker red spike on the end of them, with the larger set's spike overlapping the smaller set of barrels.

Technical
Being a Stacker branch, two barrels fire at once whenever it fires. Due to its Rainmaker heritage, its bullets also accelerate.