I know this sounds feddy, but bonus if they're improvised, cos that shits awesome.
One of my favourites is the PRIG - an IRA made grenade launcher that used a packet of biscuits to offset and quieten the backblast.
I also made a crossbow that fired nuts and bolts using wood and resistance bands. Not a weapon of war and would never be lethal, but also probably not very legal where I'm from. I just used it to plink cans in the back garden when I was bored. Was actually quite a scarily powerful and accurate device for how crude it was.
Nukes
My actual answer is the sabre, whenever I see one in a HEMA video it always looks so fluid.
thoughts on sword breakers?
They seem really cool! I know they're historical and I get how they'd work but I've never actually seen one in action, so I think I'd have to see how reliably they can fuck up a sword to form an opinion
Apparently they're not actually for literally breaking a sword - more 'breaking' in the way you'd break in/tame a horse.
I've seen videos of people using them on YouTube, in HEMA and in guided demonstrations, and the strategy is to let the blade catch in the teeth, and then pull downwards. This locks the blade into the swordbreaker, and allows you to pull it out of the way, and them towards you, allowing you to then counter with your other hand (and weapon).
Another one you might enjoy is the Plumbus. What a goofy gadget.