Short form, there is no one consistent appearance for any weapon. Clan weapons can sometimes be similar, but mainly within the original 3050 Clan Omnis. What I find best is to keep all the parts from your mods, and build up bitz.
Some general rules apply - AC20s & variants tend to have wider bores and shorter barrels, Gauss Rifles tend to be longer (except for the Pillager & Thunder Hawk, see what I mean), UACs have multiple barrels, PPCs may show a coil-like effect, or feature glow panels, but for every trend there's dozens of exceptions. Find a part that looks like what you want, and use that - there's no PPC Gestapo out there ready to crash your house & confiscate your minis ;)