I'm working on a mod which turns hitscan bullet weapons into projectile weapons, and one thing I noticed when comparing the projectiles I made to the ones you made is that yours are somewhat slow, and hard to see. they have no light or shadow defined, and are shot at speed 1.

How come it is even my problem?
Have you even looked into vanilla prototypes on cannon projectiles, before writing this?
My autocannon projectiles are just directly cloned (with very slightly modification) from vanilla cannon projectiles (at 0.16 times). I just check 1.0 vanilla cannon projectiles and they are staying the same. I have NOT downgraded the graphics from vanilla ones. Therefore, it cannot be specific to my autocannon projectiles.
If my autocannon projectile is to be complained, then the vanilla cannon projectile should probably be blamed first.

EDIT: If the vanilla definitions and graphics are changed, I will stick to the new changes.
I am not a graphics artist. And it is not my duty to improve/correct vanilla graphics.

It's just thst bullets are supposed to be faster with reduction of size, no?

It may be that they don't have any lighting, in comparison to ks combat projectiles...

Any evidence or scientific proof that autocannon shells need to be faster than cannon shells?
What is your opinion based on?

Cannon shells, bullets (and missiles, grenades, etc) are all different types. They are not directly comparable.
You cannot use comparisons between 5.56 mm, 7.62 mm and 9 mm bullets to assume (auto-)cannon shells will be performing similar. It is a bad analogy.
The hypersonic ICBM is a perfect counter-example to your "comparison of different types". The bigger projectiles are not necessarily slower.
Comparisons can only be made between the same type.

From IRL facts:
Rheinmetall Rh-120 (120 mm tank cannon of M1A2) has muzzle velocity of 1,580 to 1,750 m/s, while M61 Vulcan (20 mm rotary cannon) has muzzle velocity of 1,050 m/s. This is opposite to what you think about.
This is somewhat very reasonable. Tank cannon is designed to fire kinetic shells to penetrate enemy armour. It favours high single shot damage than a high fire rate; which is relative less concern on heat dissipation.
In contrast, autocannon is designed to fire at high fire rate. Most often its shells are on light armoured targets, so penetration is not a concern. And its Gatling design means heat is much bigger the problem. So it will use much less gunpowder in ratio, leading to lower muzzle velocity and shorter range.
So there is absolutely no reason that autocannon shells are anyhow faster than cannon shells.
(It is even "unfair" for autocannon shells to share the same speed as cannon shells, but I simply skipped it to prevent mass balancing. A lot more things need attention if having to go that serious.)

From game design persepective:
Cannon shell speed are from vanilla. And honestly I do not see any problems when firing that.
When a mod (KS combat) overbuffing its projectile speeds too high, then claiming other projectiles being too slow? I cannot agree with this way of thinking...