Currently this is my only single shot.
16.25" .35 Rem
I don't own one, but this is the coolest single shot EVER.
M79 Grenade Launcher
Anything else is just.... Inadequate!
Springfield 1868 trapdoor. If only .50-70 handloads were easier to find...
Russian Vostok CM-2 .22 LR. with a Simmons SII 36x power scope. It can do 10 rounds into a 1/2 inch hole at 50 yards.