A bit bigger than 12 gauge
If you ever feel the need to launch armour piercing shells weighing 885kg and loaded with 18 kilos of TNT a little over 35 kilometres out to sea then I know where you can find exactly the device to do it. 885kg by the way is about the weight of the current Fiat 500 car. Just down the coast from us at Azohía there is an abandoned, but well maintained, fortress that houses two rather fearsome looking cannons. The fortress was constructed between 1928 and 1936. The guns themselves were built by Vickers. Their barrels are 17 metres long with a calibre of 38cm. Not quite wide enough to launch that Fiat but pretty big nonetheless. You're going to need 20 pals to help you fire each gun and you'll probably need to fill in some official paperwork, this being Spain, but it would make you the envy of anyone who thought they were pretty cool because they owned a pair of Purdeys. Fabulous location too. Nice bit of the coast.