I've recently been getting a few fan-made supplements for the older Shadowrun editions, as while I majority play/run 4th edition I like to make use of fluff and other things from the earlier editions. In the course of this I found a few custom-designed guns for 3rd edition which lead me to the firearm design rules in Cannon Companion.

I was wondering if there were an updated set of rules (either fan-made or official) for designing and pricing custom firearms for fourth edition. I could try and convert the rules from CC, but I'm not generally that good at such things and there are sufficient changes in general statlines, let alone rules, that make it quite a job that I'd rather not do if there's already a resource out there for me to make use of.

Granted, 4th edition has practically every kind of gun you'd ever need, but I'm currently running a high-level corporate game, where my team are essentially playing the special ops of company men, and I think it'd be nice to be able to throw some experimental SOTAtech at them once in a while, be it armour, weapons, vehicles or drones, or anything in between.

So if there are potential rules for custom design of equipment (pre-modification as presented in Arsenal) I'd be really appreciative if people could point me in the right direction! ^^

There's nothing official, not have i seen any fan made.
But it shouldn't be too hard to eyeball the prices and availebilities for guns based on the guns in BBB and Arsenal.

I'm assuming you want rules because you want some semblance of balance. I don't think a complete system is really necessary for your purposes.

Just take a pre-existing weapon and buff its DV, AP, or RC by a few points. Give it a different firing mode. Add some custom feature not available to other weapons in its class. Increase the cost by 1000% or so. Whatever you want to do, just do it. Then "file off the serial numbers" by changing its name and the description of how it works. That's all the rules you need.

(I have a good feeling that's how they designed weapon stats in the first place.)

There's Shadowrun and Firearms, made by Raygun, for SR3. It wouldn't be too hard to convert it to SR4.

There are already a good deal of weapon mods. I'd think if you wanted to spend some time and build a bunch of extra weapons, give them decent names and all that, your players would appreciate the added selection. It saves them the time of having to customize their own weapons and gives you a list of cool, pre-modded weapons.

I know that my last character I spent a good hour or so customizing a revolver. It had several mods that I worked into it. I'm sure that if I had a couple dozen of those, pre-made and just had a new name and price for it, it would be useful to some.