I love using the field catalogue to do special inventories in shops. Like in Ingacio's Gun Emporium, he has mainly small assault gun from the core rulebook and also this Crewmate, a fully automatic handgun made to use in cramped quarters that would give you +2 to your gun combat check in these conditions from its ease of use.
I use it as a box of suprise my players don't have access to, same with the JTAS inventory.
I also like the knockout rules and lo pen wrapons, but that's because I, as the DM absolutely hate having players or npc shoot one another and doing no damage round after round, but I also understand the players desire to use the biggest badest armor available to them (which is often cloth armor with a trench coat, and a extensible boarding shield) and so the knockout rules were a good option for me for guns to do something without scaling to AP ammo and military-type guns and one-shooting a player by accident with a lucky roll.
But I wouldn't use the field catalogue to try and recreate a gun from a book I want to modify. I'd just modify the gun with what makes sense with the other guns in the csc. But not having the swordworlds book, if what you want is an assault sized gun with AP 4 and 20m of range, I'd use the assault shotgun from the csc, and use AP slugs, it would be 50m range, 4D damage, AP 4 and auto 2.