My tip would be to stop the problem happening before assembly. I too like making up plastic models - especially of tanks - at a fraction of the cost of metal ones. However, main gun barrels in plastic are especially prone to sag over time - so........to prevent this from happening, I usually insert and epoxy glue, a metal rod into the two halves of the barrel whilst assembling. I used to use old knitting needles ( until La Commandante caught me!), but have now sourced brass rod from a UK model shop - and now use that instead.
Brad's cure looks to be a similar method - which will mean dis-assembly of your existing model. Take care - as they can become brittle.
Long-term cure - think ahead - and stop it happening for the future when building? johnnybach