The little foil dental X-ray lead are actually quite hard. Due to their thin construction, they feel like soft lead as they are very flexible, but indeed, balls or ingots cast of them 'clink'. With most Dentists pretty much going digital now, even those little lead packs are becoming hard to find. They make perfect lead for modern handguns though.
Frosythe wrote of using Murcury for hardening lead for balls back in 1860. After reading his book, I tried mercury for hardening my 14 bore rifle's balls and found it worked somewhat but was not as good as simply adding antimony (WW) or tin (50/50 solder). Murcury did harden the lead some, but also made the balls slightly heavier than straight pure lead, rather than lighter as tin or antimony do.
A certain Englishman Army officer, a General Jacob developed a mechanically fitted bullets and moulds, which produced fins on the bullets to fit rather deep similar 4-groove barrels wound in a rapid twist (up to 22") in a SXS rifle. These rifles had long ladder sights for the military use in blowing up ammunition wagons at long range. It worked - somewhat when copper tubed bullets containing detonating compounds and powder were loaded. With these, he which he once blew up an ammunition wagon at a range of 1,000 yards.
In trying these copper-tubed explosive bullets, Forsythe found they are very poor at killing game as they needed to strike heavy bone in order to detonate, so he developed his own designs, which worked a treat on Tigers. Also - he found they worked only down to about 14 bore, being the smallest useful explosive bullet for dangerous game. Different loads were used for different game. These fell out of favour when breech loading guns of sufficient power were developed for shooting dangerous game. Then, hardened bullets could be used for deeper penetration than even hardeded round balls provided.
Samuel Baker once wrote in talking about a 14 bore rifle: "4 1/2 drachms (4.5drams = 123gr.) will drive a ball that size through both sides of an elephant's head, and 4 1/2 or even 5 drachms (136gr.) may be used on such a rifle without any very unpleasant recoil, and without any bad effect on the accuracy of the ball, but the reverse. But- great and ample as such penetrations are, vastly greater may be given to the spherical ball by simply adding a little tin or mercury to the composition of the ball: which does not injure the grooves of the rifle in the least, as they ought not to indent the metal of the bullet in any direction, the spiral motion being given by the patch alone."
Writers of the day misused the apothecary's weight nomenclature ie: drachms, instead of the word drams, which was undoubtedly meant. 1 drachm carries 60gr., whereas a dram is 27.3gr. weight.
Baker was writing about Forsyth's style of rifling, regards the ball not having to impress in the rifling due to shallow grooves & narrow lands "almost knife-edged" with 1 turn in 104" or as slow as 1 turn in 12feet - a 1/4 turn in the length of the barrels most commonly used in side by side rifles was 26", hence the 104" rate of twist.
Later - as in 1860's, the Whitworth hexagonal rifling was developed for military rifles, shooting a hexagonal shaped, paper patched bullet. These .45 calibre Whitworth rifles out-shot the then-current hollow based conical shooting Enfield of .577 calibre at long ranges.
Due to their complexity, and propensity to foul, they did not catch on and although inferior in range, the Enfield rifles remained in use by the military.
