Lower gravity on the planet

Projectiles start falling to the ground as soon as they are launched. On Earth this means if you want to lob a boulder onto your enemy, you need your catapault/trebuchet to get the boulder going fast so it can cover the distance between you and your opponent before it finishes falling to the ground. On an alien planet with lower gravity, the projectile will fall to the ground more slowly so it doesn't need to go as fast. If it's going slower it will still reach your opponent. So your aliens pick up big boulders and throw them, but they move slowly through the air like astronauts on the moon:

https://youtu.be/x2adl6LszcE?t=95

Frisbees, with explosives. The thrower presses a switch, or pulls a tag, or does something else which does not affect the weight distribution of the disc before throwing in order to arm the device for real use. Practice can be done with unarmed discs, or more frequently, with imitations made to have the same weight and mass distribution.

A well-thrown frisbee can travel quite a distance. They are sufficiently slow moving and big enough to be seen that Ultimate Frisbee players can often effectively run to dodge (or more commonly, intercept) the flying object.

I have seen humans sent to the hospital with serious injuries from even non-exploding hard plastic frisbees they weren't paying sufficient attention to striking them the wrong way in the head.

A literal American football, rest-of-the-world football, baseball, etc. could also work, with launching mechanisms traditionally used in those sports. The arming device could be set so that it won't explode on the person launching it, and the exploding part only becomes active after the launching shock and a certain amount of flight.

While only the most skilled local warriors might have accuracy at that range with a football type weapon, unless they have some special biology, a moderately skilled trained humanoid could probably get a frisbee or baseball reasonably well targeted at that range.

You can even assume the locals have a sport in which they use something very much like the object which is used as a weapon when manufactured with explosives. Then, they have a well-trained military capable of defending the peaceful tribe without having to dedicate a lot of specific time/resources/etc. to defensive war preparations.

If you need to have a launcher, I'd go an exploding baseball type object (think about pop fly ball trajectories for ease of dodging if needed) combined with something like a water balloon launcher, or a traditional catapult/trebuchet with a blunt stone, burning ball of wood, etc. We humans had & used those centuries ago. Also consider Lacrosse-style basket-on-stick ball launchers and/or disc launchers like hand held traps, used to launch clay "pigeons" in shotgun shooting when non-handhelds aren't available.

Semi-related: A very fast baseball can also be very dangerous, but not as readily dodgeable and therefore not as good a fit to this question as a slower baseball that merely explodes at the end.

Your 'projectile' is a swarm of poisonous insects

Or, some sort of living creature. Or, like, nanobots. The aliens keep them in some sort of handheld storage device. When they want to fire, they release the swarm. Tiny, extremely poisonous insects/creatures/bots fly through the air, and, if they encounter humans, sting them to death.

The swarm isn't really 'guided' so if you get out of the way, they sort of buzz on to wherever they were going. Easily dodged, if you are paying attention, but quite fatal if you end up in the middle of them.

Like this, except with more bees and less Doberman.

We've had those for millenia in our own history. They are called spears. The launcher is your own arm.

Depending on the spear and who is throwing it, the range you ask for is more than feasible. And if you see a piece of wood with a very sharp point coming your way, thrown by someone sixty or more feet away, you have enough time to duck or cartwheel out of the way.

The energy behind a projectile depends on mv^2. Bullets and similar projectiles maximize v because energy increases as the square.

Your aliens maximize m.

A very large mass projectile can be moving quite slowly and still do a lot of damage when it hits. The humans can see these things coming and dodge to some degree as long as there are not too may at once.

The projectiles are white-hot (or caustic or very sharply pointed or covered in some kind of a contact poison/toxin). If you fail to dodge they can do some major damage even without the piercing force behind them that comes from velocity in the case of bullets on earth.

You would need either some form of levitation, or lower gravity, to get past the fact that a slow projectile would generally fall to the ground before hitting the target.

A few years ago my group and I experimented with staff slings, a sling attached to the end of a quarter staff which is used as a lever extension to increase the power input. A quarter staff is just a stick or pole that is your own height plus a quarter. Throwing stones out to sea (from mainland Britain to the Isle of Anglesay) we found we could get around half way to the opposite coast, 1600ft distant, with the the local glacial deposited pebbles. They traveled fairly slowly and could easily be tracked but the most interesting projectiles were the bar shaped slate stones we threw. These were about 1x1 inch and around 2 inches long and made a loud buzzing noise when thrown. We also found that if thrown at the cliff face behind (this was discovered by accidentally releasing the stone too early) it would shatter into razor sharp spinning shards). Throwing small grenadillos or even over short distances, grenades would be devastating for such a primitive weapon. Reactive metal projectiles like phosphor or perhaps a gas warhead like chlorine similar but reduced in size to those used against the city of Leicester in the 1400's

grenade launcher

we are thinking so hard about this answer. a grenade launcher. you said it needs to still be lethal and it seems everyone is trying to find a way to make an instakilling slow moving object. why not make the slow moving projectile explode when it reaches its target. It could even be rocket propelled and be slow enough for people to react. or we could go with plasma rounds. a chain reaction of creating plasma out of the very atmosphere focused in the direction of the intended target.

Well if the projectiles are fired upwards they would take a long time to fall on the target, making it easy to predict where it's going to land if you can see it being fired.

The locals perform some entomological warfare by lobbing nests of deadly alien bees at the explorers, just like some humans may have done in the past.

Using slings or other such tool the locals (presumably wearing some protective clothes) throw the nests in a high arc at groups of explorers. The explorers can see the nests coming and jog away from the landing site before the inevitable angry cloud of alien bees form.

However, if the explorers don't get out of the area quick enough then they face death by a swarm of projectiles who just had their home turned into a grenade.

I'd lean toward a small ball-like projectile fired from a gun taking advantage of the Magnus effect by launching it with a lot of bottom-spin. (You can see the Magnus effect by comparing the basketball dropped off a dam without spin and with spin.)

The ball would need to have a skin with a design on it to magnify the effect, but thin enough that it bursts on impact with the target, releasing an acid that "burns" through clothing and flesh. It could be an acid vapor under pressure, for instance. You could even have the ball's skin only partially resistant to the acid so that it's eating at the ball from the inside, making it more fragile and prone to bursting when it hits its target. (The weapon would fill the ball when you cocked it just prior to firing it.) That also has the advantage of them bursting sometimes in the air before reading the target, which can still be useful to the attacker.

In terms of justifying it vs. arrows or metal bullets, perhaps there isn't that much ore and wood around, but there are abundant sources of this acid and the material for the skin of the ball. (Perhaps they both come from the same abundant plant, for instance.)

Projectiles that are accelerated over the course of their flight

Such examples of this would be the real-world GyroJet ammo, in which each caseless projectile had a small "rocket motor" which accelerated the projectile over the course of its flight, as opposed to a traditional bullet where the acceleration occurs in one very short event. These rounds sped up over the course of their flight until their propellant ran out, so it could theoretically start slower yet reach lethal speeds easily after traveling a long distance.

EDIT: I reread the originally posted question after posting this answer. While GyroJet ammo does make the projectile powered after leaving the weapon, these were technically their own, self-contained bullet

Extremely dense projectiles in low gravity

Works great on a planet with massive Osmium reserves!

As stated in another answer, the equation for the energy behind a projectile is $mv^2$. This means that (aside from physical hazards such as explosives, heat, and sharpness) there are two ways to go about increasing the power of the projectile:

• 
  Make it heavier: A truck at 40 MPH is much more likely to kill you than a piece of paper at the same speed


• 
  Make it faster: Although much more effective at increasing the energy (because it is squared), this projectile is slow

So, if we go with heavier, we'll need it to be dense. I'll assume the target energy should be that of a .230 grain, .45 caliber bullet (252 M/s, 28.35 grams). This equates to a $mv^2$ of 7,813,968. I'll assume "slow" is two seconds to cross the 20-meter distance mentioned...10 meters per second. This gives us a $v^2$ of 100, meaning that the projectile must weigh around 78 kilograms (172 lbs). That gives us around 3,473 cubic centimeters of projectile, assuming it's made of Osmium (the densest metal that exists). This could be formed into a cylinder with a diameter of 10 cm and a length of 45 cm. Of course, this would be quite hard to make into a handheld weapon, and would drop to the ground almost instantly. The solution? Lower the gravity! On a Moon-sized planet, this would be only 13 kg. Add a new explosive to propel it at the right speed, and make a few tweaks to the gravity, and voila!

Why would these be used in the first place?

The land is covered in hills, and the gravity fluctuates enormously depending on altitude. Bullets and arrows are hard to aim, since you have to be able to tell how they drop depending on where you are. Problem is, they travel so fast you can't see where they went. Imagine some sort of fog, or really tall grass. However, this slow projectile is very easy to see, allowing the attacker to gauge their aim after one shot. As an additional bonus, the durable and easy-to-find projectiles would be easy to scavenge after the battle is over, allowing them to be reused.

Everyone is offering different methods without any justification why that method exists. Without a plausible explanation, a slow moving launched projectile will break suspension of disbelief. Here is a system that can work...

Your aliens are slow moving, strong, and have exterior shells. Thus they need tools to speed weapons to be able to penetrate shells of their enemies. With their strength, they use huge arrows (made out of steel) that move slower than human arrow to kill their enemies. They do not need it to be faster as their enemies are not too fast either. But a human is fast enough to evade these arrows. However, the size and sharpness of these shell destroying arrows is very deadly to humans.