This list only includes weapons that are natively patched by Combat Extended itself, so there might be more.

.470 Nitro Express: "Thrumbo Express" Big Game Rifle from Rimsenal - Marauders[ludeon.com] (Marauders is only in the 1.0 Dropbox link, not 1.1)
.55 Boys: British Boys Mk I Anti-Tank Rifle from [K4G] RimWorld War 2
.600 Nitro: None
.950 JDJ: None
2 Bore: None
30x165mm: None
30x113mm: None
20x99mm: None
20x42mm grenade: Launcher Pistol from Star Wars - Factions, Explosive Zebra Shotgun from Epona Race Renaissance, BZ-4 + I2 Knight from Urbworld Weaponry: Caseless, and Neopup PAW-20 from Combat Enthusiast's Collection (Continued)
20x110mm Hispano: PnL Portable Cannon from Paniel the Automata and NTW-20 from Combat Enthusiast's Collection (Continued)
20x128mm Oerlikon: HST from ADE Advanced Turrets and Anti-Material Turret from Turret Collection (Continued)
15.2x169mm: IWS-2000 from Girls' Frontline Weapon Pack
13.2x92mmSR: None

[EDIT] The "Explosive Zebra Shotgun" is actually called the Zebra Auto Grenade and is only included in this older version of Epona Race.

Epona Race Renaissance has its own Combat Extended patch from an older Rimworld version, and with that installed, the Eponic Steamgatling will fire .55 Boys rounds. However, I can't guarantee the patch still works in 1.4.