I spoke with the man who owns Gillmor Ordnance Ltd. today, (thanks Intoodeep) and it is his foundry that is (maybe 'was' would be more up to date) supplying C-mania with most of the bronze guns he offers for sale.
The man's name is Bob Gillmor, and he seems like an amiable gent, so I suggested that it would be to his benefit to become a sponsor here. He got GBO-BPM&C up on his pc as we spoke, so maybe he'll follow through with a contact.
This isn't meant to be a commercial, that wouldn't be right, unless he became a sponsor, but here is some information he provided: There is no discount given on a group buy, he casts cannons from .38 caliber to full size, also a 24-pdr coehorn, and as everyone would probably surmise, C-mania marks the prices up considerably.