You often see them on shunting rifles. Like the Shuntsman or Shaitch W 100.
Shaitch?! U sound like bloody Sean Connery!
"corners should be round" Theo Evo .22/.177 - Meopta 6x42, DS huntsman classic .20 vortex razor LH 3-15x42 under supervised boingrati tuning by Tony L & Tinbum, HW77 forest green - Nikon prostaff 2-7x32 plex.