Try .177
Best thing is to 'suck-em-&-c' Try as many (good quality round head - no gimmicks) varieties as you can. JSB is a good starting point over a minimum of 30 yards indoors. Ask other HFTers a the clubs what they shoot. I think the most popular at Rivi is JSB but Im no anorak so I cant tell you which is the best size/batch number.
Welcome to a world of grief.
ATB
Ian