There are a number of weapon features that are currently only available on weapons that shoot bullets (like spinning or aiming down sights), and you can't make these weapons shoot projectiles by editing params. While it is likely that this will be fixed by Croteam eventually, in the meantime Bullets to Projectiles mod can help out.

The mod's mechanism is simple. Bullet-shooting weapons have an option to make bullets they shoot not hitscan, but physical projectiles instead. But you can't customize these physical bullets. The script in the Bullets to Projectiles mod "catches" these bullets and replaces them with projectile of your choice. That's it.

First, make sure you need to use this mod to implement your brilliant weapon idea. If you don't need any of the following features in your weapon:

• Ability to aim down sights;
  
• Ability to shoot many projectiles at once like shotgun;
  
• Ability to spin your weapon like minigun;
  
• Ability to aim like sniper rifle;

then you definitely can make your weapon without the mod.

Ok, now that you've firmly decided you need to make, for example, rocket-shooting shotgun, let's start.
First, prepare a projectile your weapon will be shooting.
Second, make you weapon as a bullet-shooting weapon, with usual hitscan. Configure the spread, number of bullets and such. Don't make very large spread (a bit bigger than double shotgun's spread is fine), because script can not work properly then. Then, in weapon parameters, change "Bullet type" to "Physical, flying":



Your weapon should shoot yellow glowing bullets now. Now you need to make the script replace the bullets with projectiles. But before you do that, change the speed of bullets in weapon params to zero. After that, create a "Script entity" on any level, and double-click on it to open text editor on the left. Paste the following code into it, changing "PathToWeaponParams", "PathToProjectileParams", ProjectileSpeed and X,Y,Z with values for your weapon:


X, Y and Z are corresponding values of Position vector in "Rendering params" of your weapon.
For example, for Flak Cannon it looks like this:


Now go to "Entity" tab while your script entity is selected, press on (local) -> Save As and save this script into each of the three folders under any name of your preference:

/Content/SeriousSamHD/Scripts/CustomWorldScripts/
/Content/SeriousSamHD_TSE/Scripts/CustomWorldScripts/
/Content/SeriousSam3/Scripts/CustomWorldScripts/

(saving it in each folder makes it work in the levels of the game you saved it in).

That should be it. You don't need the script entity anymore, only the files that you created matter. Now start the simulation and shoot with your weapon. If everything is done right, it should now shoot specified projectiles (with specified launch speed) instead of yellow bullets.