As above really. .177 and get at least 6 different types of pellet and check for accuracy/grouping at your chosen range.
JSB exact 8,44gr .177 4,51 head
JSB exact 8,44gr .177 4,52 head
JSB exact 8,44gr .177 4,53 head
Maybe try some of the 7,9gr variants as well.
Clean the barrel between each pellet type and also be aware that you can get different results from exactly the same pellet but ones that have different batch numbers.