I did wonder about this but I've found neodymium magnets 20mm dia and 20mm long which have a 15kg 'pull' if you had two of these with the same poles facing each other in a suitable size tube wouldn't that equate to a 30kg 'push' ?
Not sure if that would work but I have a couple of tiny ones here ( 4mm dia x 8mm long ), no idea of the 'pull' but it hurts if you get your fingers between them when they attract and if I try to push the same poles together it's virtually impossibly without them pinging across the room
I might have to rig up some sort of test rig first before I spend too much time cannibalising that old rifle.
Besides the lure of BBS cheese biscuits beckons.
Norm