Many moons ago i've spent time at a place where got to watch this local Park ranger do something quite funky:He chucked an axe head down in a vice,and drilled a 1/4" hole in the butt of the handle that faced up vertically.Then he inserted this tiny funnel in that hole,and for several days kept topping it off with linseed oil(don't remember if he cut it with solvent or not).He kept it going till the oil came dripping out the other end at the head...Then he sealed both ends(with some oil-compatible goop,forgot what it was he used...).
I was very interested,never having seen the like.And at one point he demonstrated to me the efficacy of the method.With a 6 lb splitting maul(he was doing it to all the handles he could get his hands on

,he deliberately overshot a stump,kinda scary-hard,and the handle not only didn't get damaged,but visibly flexed and bounced,rubber-like...The degree of deflection it sustained was impressive.