I agree with Huvius leave it, it will take rabbits easy - H&H is too good to mess with
In the UK, Henry Krank sells cases, Andy Allwood sells .300 bullets (.308" swaged down) very reasonable. [All on Google] 2.5 grains Bullseye is a popular load
Colin Greenwood's "The Classic British Rook and Rabbit Rifle" ISBN 1-861268807 is only 19.95GBP and well worth it if you are getting interested in these little rifles and includes some loads.
There is quite a bit on loading etc. on the YUKU British Militaria Forum
Please do not take offence, but it needs to be on your Firearms Certificate if you shoot it
As with the other post, pics would be nice if you get it
TH44