I take it you've never seen my conversions...
Mapping a separate texture over a plain barrel can give the illusion of complexity.
Or you can model some med poly ones, or cheat and use weapon pofs over your fire points liek Steve-o's weapon pack! Also Nuke Mod's weapons add on the pof.
NUkemods ROTATING gatling guns added to pof via table (Steve-O's Lancer/Nuke's ttable)

Or you can extrude and bevel flat faces/boxes and use a hole texture or glow map to make missile racks or surface lasers evident...(Aldo's banshee)

Or you can use a combination of slightly more model detail and more complex texture... (Kreeargh/UW's Model, my kitbash and re-texture)

Depending on what you want seen you can make better nozzels for under 500-3000 polys no problem
I like shaped ports with either flaps or solid nozzels.
("Gunryu" tribute to Captain Tylor - Not exact, but it was a happy coincidence from the Starshatter Community, exact artist escapes me atm).

So I hope some of these examples broaden yuor horizons...