Q:

Slow moving projectiles from a hand-held weapon - how do they reach the target?

Human Earth explorers land on planet X. They are there to survey it. They are unaware that there is life on X until ...
The locals are very aggressive and fire on them from a distance. The thing is that the projectiles (which could be bullets or arrows or whatever) travel very slowly. So slowly that the humans can see them coming and dodge provided they are looking in the right direction.
If the humans are hit by a projectile then it penetrates them and can be fatal.
Question
How can I scientifically (and not magically) reconcile the slow movement of the projectiles with the fact that they can reach a distant target and be fatal?
Assumptions
The action takes place away from civilisation. You can imagine scrub-land with low hills.
The weapons are hand-held by aliens that can be considered similar to humans.
The projectiles are fired from the weapon, not thrown by hand.
Once a projectile leaves the weapon it is not powered (i.e. it's a bullet, an arrow, etc.).
Any reasonable science-based assumptions may be made about the atmosphere or other physical characteristics of planet X.
The range of the weapons must be at least 50 metres. They must be able to work anywhere on the planet.
Humans must be capable of seeing and dodging an incoming projectile from 20 or so metres away before it reaches them. The projectiles must therefore be slow-moving and big enough to be seen.

A:

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

A:

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.

A:

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.

