Danged if I can remember what size cap it took.
It was a loverly Thompson double trigger target model, with vernier rear sight and globe front.
It was quite awhile ago and, like a big dummy, I sold the rifle.
(Fill in sounds of head repeatedly hitting wall).
I do remember, however, that the load was 45 grains, and only lightly compressed.
Something about 777 requiring some air space between the grains for effective ignition, maybe??
Lock the doors, they're coming in the windows.