you can use a berm or bank in the UK, AGB have guidance but it is really too vague to be fit for purpose. They need to revisit this and put some numbers in with regard to elevations, energy levels of projectiles etc, not just say that compounds have an overshoot of x because of the mechanical release aid. In your case a 1m bank with a 2m fence would cover the beaten zone for 100yds but I'm sure that you wont find this in their guidance, especially as they outsourced the writing of it to a company that doesnt actually have any published expertise in such matters.
AS for swinging a cat, it is a naval phrase but was in use before it was first associated with the cat-o-nine -tails. There is also a suggestion that cats were placed in bags and shot at by archers so not enough room to swing a cat would mean no room to place a target somewhere.
Letting the cat out of the bag refres to the practice of selling piglets in sacks to prevent them escaping and unscrupulous sellers putting cats in the sack instead. A bad buy being the pig in a poke, ie the unseen piglet/cat