There is ‘big bore’ and then there is the 2 bore. Schroeder & Hetzendorfer make a cartridge called the S&H 2 Bore Blackpowder Express.

These photos are amazing

.700 nitro express next to 2 bore and 3500 grain bronze solid

.500 S&W, .45-70, .700 Nitro Express, 2 bore

The caliber equivalent of 2 bore (‘bore’ is generally interchangeable with ‘gauge’ in regard to caliber) is 1.326″ or 33.68mm! The bronze solid bullet weights 3500 grain / 1/2 pound / 226.80 grams!

While technically fired from a rifle these are more like massive shotgun slugs: a heavy projectile at a low velocity. The 2500 grain bronze solid leaves the barrel at 1500 ft/s and with 17487 ft·lb of energy.

Apart from the nerve destroying recoil, heavy rifle, heavy ammo and expense these big bullets slow down dramatically after entering the game and hitting bone. A problem when hunting elephant.

They even come bigger than this. The 1 bore has a caliber of 1.67″. The A Guage/Bore, about 2″, was used for punt guns.

More info at Schroeder & Hetzendorfer.

ACE has an interesting article on the 4 bore.

UPDATE

Jay Schroeder emailed me the pricing for for the brass and bullets:

Cartridge Cases are $495.00 per 10 pack shipped.

Copper FN Solids are $295.00 per 10 pack shipped.

All components are USA made by Americans from American material.

Although it may seem expensive, you are probably not going to purchase more than one set of brass.