Can anyone tell me what size tubing to use for the main and five inch secondary guns that won't be used for firing?
Pretty much whatever looks good to you. Offhand, scaling down the barrel of a 5" gun would end up somewhere around 1/16" diameter rod I think?
Try the spring out of an old oil seal, available free from your local mechanic. They can take a hit most times with no damage, appear hollow and are easily glued in place.