20mm cannon barrel replacement

Hercmech

the cotton swab (Qtip) that has the plastic tube...not the paper ones. Heat like sprue and stretch to the right length/thickness and cut using a sharp blade

You can also make turnbuckles for WW1 aircraft usining this method. 

Great responses.  I will try all of the suggested techniques.

Thanks again!

...syringe needles...

Thanks to a Hans von Hammer, I have tried the following and  it works very well. Get one of the cotton swab (Qtip) that has the plastic tube...not the paper ones. Heat like sprue and stretch to the right length/thickness and cut using a sharp blade... voilà a nice thin plastic tube. You can also flare the ends really easy which simulates the cannon great.

I model in 1/48 or 1/32 and use brass tubing. At 1/72 you may probably use a syringe needle for the barrels.

You usually do something like that out of stretched sprue (you know, you take the "frames" the parts come on, cut out a straight piece, heat them over lit candle or a zippo lighter and then pull a little so that they get thinner...) - drill the ends out a little and there you go! Good luck with your project, have a nice day

Paweł