I'm now selling these on my site and they have replaceable heads!!! https://www.3dlocksport.com/products.php#!/Bump-Hammer/p/155651360/category=39551008

I bought a Sparrows Bump Key Hammer. It is a great piece of equipment. I wanted to see if I could replicate it and make one cheaper. From Sparrows the cost of the hammer is about $12 USD. This hammer costs about $5 USD in materials to make.

See a video of me using it here:

If you are looking for the mini version it is here: https://www.thingiverse.com/thing:2707828/