I am not a great fan of steel BBs ... too much rebounding etc.
So I tried the 4.4mm copper coated lead balls I have and compared them to standard steel BBs in my 'Witness'.

Basically the steel BBs were much more accurate with a group size maybe 2.5 times smaller than the 4.4 balls. Maybe I am not too surprised as 4.4mm is smaller than the steel BB size so there is more room for inaccuracy to bite in the barrel.

I have not tried Gamo lead balls in 4.5mm as I am sure I read that they can jam the barrel ... the Makarov I once had certainly did not like them too much but that was down to a deformation issue rather than the barrel.
I have heard that 4.45mm lead balls are good for the Witness. Can they be found in the UK?

Cheers, Phil