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??
Walt Kelly, alias Pogo, sez:
“Don't take life so serious, son, it ain't nohow permanent.”