Pacel Force 48hr service is the only delivery service other than RMSD who will take airguns
This ^^^^^^^^
Another option when using RMSD was to seperate the stock from the action in order to fit and pack it into a box that was within the permitted size.
I used to use Parcel Force 48 before the new ruling but at the risk of not being able to claim if the item went missing as it was not a permitted item.
Good Luck.