Sorry to hear that about the guns, but perhaps you can use them for a shore battery, or fort onshore. Even pirates would drag guns ashore when careening their vessels, and make a sand, or dirt berm to protect them and their vessel, while they were most vulnerable.
They'd also be good on deck too, for your vessels.
If pressed, you can always pull out the Dremel, and cut them down to size, but that seems a waste, since you have full barrels and the carriages too.
Here's one company that sells half barrels:
http://www.castyouranchorhobby.com/cat--Brass-Dummy-Gun-Barrels--Brass+Dummy+Gun+BarrelsNot sure what size you need, or your budget, but it's one place to start.
Never dealt with this company before either, but there are lots of suppliers out there, with different offerings, types of cannons (shapes), and sizes. It seems as if there are more in the UK than here in the USA, but you should be able to find a decent domestic supplier with a bit of searching.
Look for parts/fittings for wooden ship models in your searches.
They seem to be either called dummy cannons/guns, or half-barrels, by many companies.