What is grapeshot? Grapeshot is an anti-personnel load for muzzle-loading cannon of the gunpowder era. It consisted of a either a bag of musketballs, or a stack of musketballs set on iron rings. The result was very like a super-sized shotgun blast.
Did cannons originally shoot grapes? Well, no, of course not. Actually olives were the first ammunition of choice in the middle ages, when not firing large rounded stones. Cannon first appeared in the Middle East, and then in the Byzantine Empire which hugged the Mediterranean, where olives were plentiful and readily available. The large pits worked as effective penetrating projectiles, and the oleaginous flesh of the olive readily ignited from the flash of the gunpowder, and inflicted painful burns in and around the entry wound. In the right conditions (especially at close range) the victim's clothing often caught fire.
So where did the "grape" come into grapeshot? As the use of cannon spread into central and northern Europe, where olives have to be imported, a change of ammunition was required. Acorns, while readily available, have very unsatisfactory flight characteristics, and are too light to be an effective ammunition. Peach pits were better, but they tended to split in flight. The humble grape, it turned out, was the best replacement for the olive, though the smaller seeds rarely produced a fatal wound all by themselves. The sugars in the grape, while not producing the "Greek fire" effect of a burning olive shot, still tended to cause smoldering and flaring in the victim's clothing and hair. The yeasts on the grape skin introduced persistent and disabling infections in the wounds; and artillerists soon learned that one grape hitting a soldier in the eye was enough to put them out of the battle for at least the day. In England and northern France, where English speakers first became familiar with the innovative multi-projectile loads, it was grapes that were most commonly used. Hence grapeshot.
I thought grapeshot was made of lead? Yes, from the mid-1600s on, lead and iron balls replaced the reliable old grape. While iron did rust, both kept their shape for years at a time, whereas grapes would have to be replaced after just a few weeks on the ammunition wagons. The metal projectiles were also just as readily available in the winter, which was not the case for grapes. Lead and iron balls traveled much further from the mouth of the cannon, and they delivered a much harder blow on contact. Flight characteristics were superior to both grapes and olives. On the downside, the burning effects were sacrificed. And so was the option of eating the ammunition if rations were low.
Why aren't grapes used in warfare today? Grapes, olives, dates and "all tree fruit projectiles of any kind" were outlawed by the Geneva Convention on the Amelioration of Wounds in Time of War. Indeed the only vegetables permitted as weaponry are root crops (potatoes, yams, peanuts, cassava, etc.).