Can anyone offer any air rifle advice please?

[Mike, Lord of the Spammers!]

I seriously hope this "target practice" you speak of does have anything to do with me running naked through the local park at dusk again 

Aw gwaaaan. Be supportive. You know that a moving target is the best 

We'll want piccies

As for the relative merits, Well .177 shoot's along a much flatter trajectory, so range estimation is less critical

.22 have generally better knock down power, but at the expense of a rainbow like (relatively) flight path

[Styerman]

As a general rule .177's group tighter . When the kill zone is small , group size matters . 5mm is another option , only pellets are tuffer to come by . Forget knockdown power , you are restricted to 12ft.lbs. ( a .22 rimfire IIRC is 73-90 ft.lbs. ) . It's all about accuracy , otherwise you would all be using the old skool .25 airguns .

Chris