Welcome to Librarium Online!
I've been thinking with my list that I'm gonna run Duke Sliscus with a squad of Trueborn in a raider with splintercannons and Shardcarbines. Unfortunately I can't really think of a good way to convert the weapons. A friend suggested I use the old Splintercannons but I don't really have any sort of access to them. My main thought is to maybe cut the barrel off the splinter rifles about halfway down (so the length of the gun in total is about 2/3 of what it was. I think it could work, but I wanted to hear how others were doing it.
The enemy's gate is down
Be lazy. Use regular model and just say that "my army is one of deception, it uses regular gun casing but really inside is an array of modified and complex attachments which work like a shard carbine"
“When I give food to the poor, they call me a saint. When I ask why the poor have no food, they call me a Communist.”
I looked a picture of the shardcarbine in the codex and it looks shorter and stockier. Converting it to be like that will help. I don't think you'll need to be exact. If your opponent can clearly tell they are different then that may do. I've done similar things with my Tyranids. I have a Hive Tyrant with huge claws (Hive Tyrants can't have the crushing claws upgrade) that are meant to be a bonesword and lashwhip, they look different enough for it to work. Loads of bug players also make devourer cannons to show twinlinked devourers on monstrous creatures when they don't have enough of the bigger devourers (Termagant devour bits all stuck together to be bigger) and it works fine.
You can do the same thing with DE bits just make sure they are cleary something that is not a splinter rifle. Three splinter rifle barrels all together may make it look like it has a meaner rate of fire. Having a bit of fluff to go with the shardcarbines will help like they are custom amde etc. My Tyrant with big claws leads my swarm of scorpio-Tyranids, so it works. Converting your Trueborn a bit to make them look more important will also help them stand out as a special unit of some sort and not normal Kabalite Warriors.