In reality, you can't have a too dense projectile for a sling. Osmium, which currently is the most dense metal known, would be optimal. however, it would be like 800 USD for a good gland. Affordable wise lead is the best option, which is definitely not that dense, to be a problem. Not even close. Osmium glans might just overpenetrate ,if they plow through only soft tissue.. but even that would be relatively rare.

Ouch. Sounds like a lot of rifle and pistol caliber debates I hear from friends in the Army. 9mm V .45ACP, 7.62x51mm V 5.56x45mm, and so on.

My argument would be to use the models to get a range that will put your shot through the ribs and intercostal musculature, because there's a good chance of causing a Pneumothorax and collapsed lung, but without the resistance of the skull factoring in as armor with angles and penetration, deflection, etc.

Slings, like many projectile weapons, kill primarily by Exsanguination and Shock, so going through the lungs and just past the half-way mark would likely be your best bet, due to collapsing the lung and damage to high-pressure, high-volume blood vessels.

Figure out the depth of penetration necessary to get this, and you'll have a formula for energy required of any diameter of projectile, after that, it's just balancing mass and speed over that threshold, though I am willing to say you'd need to set a minimum for each to achieve you goal. I'm willing to say that larger cavitation is going to be more deadly every time, and at the speeds a slingstone travels at, you'll need to have a wider projectile. They just aren't going fast enough to induce the kind of temporary shock-cavitation a rifle or modern pistol bullet induces.

So, with possible energy a sling can dispense, you could get to a really complex model here. If you set the range based on a Sling's possible power output, you could give a table of weights and diameters that are viable for such purposes. But, that seems a lot of effort for not a lot of gain. Historic models should give us what we're going to learn, and I bet we'll learn that Boars aren't something the Sling can take on effectively.

Also, don't hunt boars without a .450 Nitro Express, .357 magnum, a spear, and a sword. Those things are mean.

(Edited for horrid spelling and clarity)

Someone needs to get a dead boar and try out some razor glandes on it. My money is on the sling. Blunt ammo I think would be a mistake.

When I chased off momma pig with all her little ones, the little ones ranged from around 25-30 pounds, (10-15 kg) down to maybe 10 pounds (5 kg).  I have no doubt that an egg sized rock that connected anywhere on the smaller and on the head or shoulder area on the larger would have incapacitated it by breaking bones and possibly rupturing internal organs.

I didn't try it because I also figured that the little piggys' screams of pain would have also triggered an attack by momma pig and that I would have lost that fight.

Somehow, the thought of momma pig eating my guts was enough to make sure I didn't do anything particularly stupid. 

This time.

This is a great article Teg! Thanks for providing the link.

I like to sling smooth 120-150 gram stones against an old brick wall from about 20 meters using balearic style with a balearic sling. The first time I did that I thought I would be able to recover my stones as I thought the stones would simply bounce.
Turns out they completely shatter on impact. Doesn't matter what mineral stone I've used.
The power of a sling is very much dependent on its user. I see many beginning slingers sling with the wrong form with sidearm style. They take their back foot too high off the ground. I believe Luis Livermore's form is the most powerful practical slinging form and I've become quite good at imitating that.