Sure, the Winter Systems Mercury Ship Laser is neat-o. And the Aztech Gauss Cannon is handy for annoying the neighbors. But the SS mode of fire really needs an upgrade for true pant soiling terror.

So, can Weapons Mods (Gas Vent, Firing Selection Change, etc) be fitted on Vehicle Main Guns?

Mechanically?

Maybe.

Realistically, at all?

Not a chance.

Vehicle main guns are meant to be GIGANTIC - think the Main Gun of a tank.

They're less "pieces" of a vehicle than systems attached to the larger system-of-systems that is the vehicle.

They can't be modified, not realistically - they're too finely-balanced to accept modifications.

Depending on what you wanted to add on, I might allow some of them. Gas vent, maybe. But the rate of fire for those large weapons is more dictated by the speed you can load at versus the recoil penalties you rack up. Firing selection change? No way. In a firearm, an automatic rifle isn't fundamentally very much different than a semiautomatic rifle. But in terms of a large ship's cannon, there's a huge mechanical difference in the way the gun works between making it single shot and fully automatic.

In terms of the game rules, I don't think any of this is allowed anyways, so it's a purely academic discussion. If I was GMing, I may allow it in certain instances (like on a vehicle mounted machine gun or something), but that'd always be GM's discretion on a case-by-case basis.

i'd allow underbarrel guns, because most MBTs have coaxial MGs.....
also maybe weight reduction and barrel length mods, but those are not really worth it.
extreme enviroment adaption on the other hand is...

weapon firing selection mod from SS to SA yes, from SA to BF or FA, no.
additional magazin would also make sense, but because main guns in SR4 have only one ammo type it is pointless.

perhaps as a factory modification, but i don't think you're going to be doing much of anything with a kit.

Semi-auto is not much different than auto-matic in terms of basic mechanics. Both use the gas to recock the weapon. Some semi autos can easily be made automatic as well, and most small arms that are full auto have a semi-automatic feature as well.

As for modifying artillery-no-I would not allow it-there's is too much to balance out b/c of stresses, mechanics being different, and game balance.

Most main-weapons for tanks & APCs and all the different vehicles that come between usually come with just about every accuracy increasing feature available that is important to the design. Short of mated weapons though, there isnt much you should be able to do without a facility to do modifications that would count as "Factory installations".

If the Striker fired it's cannon more than once, It'd probobly do backflips.

Also some mods are simply not working. Like adding gas-vents to a railgun or laser...