The posts about the .700WTF and the .950JDJ drew a LOT of hits, so apparently this is a popular topic. On that last post, one commenter said:
Can we get a 1.000 OMG? Going once… going twice…
Which was followed by:
Screw round numbers.
Make it the 1.001 OMG just to break the barrier firmly.
Already been done, boys, in a rifle you actually fire from the shoulder, not off a bench. The 4-Bore double-rifle:
According to Wikipedia:
The name derives out of an old English practice of bore measurements in gunmaking, meaning a perfectly round pure lead ball that exactly fits the bore of the weapon would weigh 1/4 lb — that is, 0.25 lb (0.11 kg) (see gauge). The 4 bore was made to fire roundballs of 1/4 pound lead, or approximately 1750 grains (1750 gr.) and approximately 1-inch (25 mm) calibre (more precisely, 1.053-inch (26.7 mm), when shooting pure lead bullets). This varied greatly as in muzzle loader days shotgun gauges were custom made and often differed from the actual bore measurements. Commonly, 4 gauges were closer to .935-.955 calibre, pertaining to a 1400 gr. alloyed lead ball and closer to 5 gauge.
So it’s hard to say without actually examining a 4-bore, but it could indeed be 1″ or greater. Continuing:
The cartridge brass case was around 4 inches (100 mm) long, and contained three types of loads: light at 12 drams, 14 drams at regular, and 16 drams of powder at heavy load. (Note: 1 dram = 27.34375 grains in the avoirdupois system, since 256 drams = 7000 grains = 1 pound of powder. Shotgun shells are still rated in terms of the same archaic dram measurements, relative to their equivalence of smokeless powder load to a blackpowder load weighed in drams.) John “Pondoro” Taylor mentioned in his book African Rifles and Cartridges that the 12 drams (328 gr., 3/4 oz.) charge would propel the projectile at around 1,330 ft/s (410 m/s). A double barreled rifle that would fire such a calibre would weight around 22-24 lb bare, while the single-barreled version would be around 17-18 lb.
John Ross, the author of Unintended Consequences has one of these beasts. Here’s a sample ballistics table:
With a “heavy” load of 440 grains of black powder under a 1743 grain projectile, the free recoil energy of 262ft-lbs. is very much in the same class as the .950JDJ, only the JDJ rifles weigh between 50 and 110 pounds and are fired off a bench. The 4-bore doubles weigh about 25 pounds and are fired standing, from the shoulder.
OUCH.