How long is the gun? with a 75 you can use a threaded shaft with an harpoon point but if you have a large gun and you want to take longer shots then the losses in range and precision are too high.
If you spear is still sticking 10 cm out of the muzzle, then you can fix it with a grinder and a friend with a column drill and (drill 2,25 mm)… replacement barbs are always handy!
Make some test-shots if you rebuild the spear because the point of aim will change.
Ciao!
