Are there any rules on creating new weapons? Arsenal does a pretty good job with weapon modifications, but what about a whole new gun?

The game is too abstract and existing weapon design is too arbitrary to support any sort of firearms design system. Take an existing weapon, tweak some details, tweak the cost, show it to your GM, and wait for either a "yes" or a "no". It's basically how the existing weapon stats were created.

Adapted from my vehicle creation method:

1. Make up what the vehicle weapon is or start with a vehicle weapon in mind that needs stats.
2. Compare and contrast the vehicle weapon you're designing with some like it that are in the rules.
3. Assign stats that you think make sense and give you the vehicle weapon you want in your game.
4. Use the vehicle weapon in your game.

You can have as many new weapons as you can imagine new weapon names.

They will tend to have the same stats and mechanical effects as existing weapons and modifications.

There's rules for making a completely new weapon in Cannon Companion for SR3. If you're up to doing some work, you could possibly adapt that to SR4. Basically, it starts with a separate weapon frame for each of the categories, you can add some design options from there, and then you can go in and add any modifications you want. The rules in Arsenal encompass a lot of the stuff you could do with the Cannon Companion weapon creation rules anyways, though.

i believe kerenshara was working on something for this. maybe she will post it later.

If I recall, Kerenshara was working on a system to completely overhaul weapons and their damage to make it a bit more realistic. I don't know if this was going to include rules for the custom creation of weapons are not.

But like has been said before, you can make up stats for anything, give it a name, and there you've got a new weapon. The real difficulty is in creating something that's balanced.

I tried to convert Raygun's Shadowrun And Firearms to SR4 but I gave up. SR4 is even more abstract than SR3. To be honest, all you need is ONE weapon of each category and mods from Arsenal.

Don't like it...!

Thought about modifying the system a bit. What about this:

Weapon DV is multiplied by 10. A fictional .22 pistol would have a DV of 25P and AP +1. Full Body Suit would have 6/2. Damage Boxes would be (8+BOD/2)x10. So... A BOD4 human would have 100 damage boxes.

Attacker: Every net hit raises DV by 10% and final DV would be rounded up.
Defender: Every net hit reduces DV by 10%.

Modifiers:

0 - 30 damage boxes: -1
31 - 60 damage boxes: -2

...

Example:
Attacker fires a .22 pistol and has 2 net hits. DV is 25 + 20% = 25 + 2,5 * 2 = 30.
Defender has 3 net hits. Final DV is 30 - 30% = 30 - 3 * 3 = 21 S

First Aid or heal spell would heal 10 boxes per net hit, instead of 1 box.

This way we would have more options to create new weapons because theres much more space for fine tuning. What do you think?

Well you first need the armorer skill. If the weapon is relatively radical or seems to go beyond the basic modifcations then a knowledge based skill on weapons design is required. If the weapon is trying to be cutting edge then you raise the extended test and require even more skills.

If you want to alter stats or combine features from the book that seem to take up the same space I would put that into the design category which means appropriate skills such as electronics, metallurgy, physics, etc.

GM fiat if you think the weapon is more powerful than the basic mods to weapons given in arsenal or SR4 core then you need to use knowlede skills along with armory along with a facility.