Because I don't know the weight of the ball, I cannot figure out the fpe. FPE has little or no relationship to actual likking power. I know there are numbers assigned to ctgs. and their loads, but the numbers mean nothing. You cannot compare a .45 2-7/8 Sharps to a .30/06, power wise, yet they produce the same fpe., not can you compare my .50/70 to a .22 Hornet's hot load in a CZ, yet they produce the same fpe. Thus you cannot compare a round ball from an 8 bore to a .375 or .300 Ultra Mag, or .300 mag, yet they produce, probably, similar fpe.
; The 8 bore is & must be, a smashing round on heaviest game, depending on how it's loaded. The others are small bore rifles that lack the pounding force of the large bores.
; Sorry, but this is the best answer I can come up with.
: Loaded with an appropriate charge and a patched, or wadded round ball, the 8 bore develops tremendous power on animals.
: You should be able to achieve from 1,300fps to 1,500fps velocity from it's round ball. I have a drawn full scale duplication of a group fired by an 8 bore smooth gun with round ball, fired at 60 yards. It contains 3 lefts and 3 rights. The group is a vertical one, 5 1/2" on centres. The group is 5 inches tall and 2-1/2" wide. Another, shot by an 8 bore rifle, had 8 shots at 50 yards in a square 2-3/8" X 1-9/16". This may have had rifled bores of 20" to 22" length. They seem not to have liked longer barrels in the thumpers.
: If you shot a true 8 bore ball, it would weight 7000 divided by 8 = 875gr. At 1,500fps, this would generate 4,375fpe at the muzzle. Out to 50 -75 yards, it would maintain quite splendid power.
: W.W.Greener's 11th Edition of "The Gun and it's Development" states " Possibly the best weapon for large game is this 8 bore with short barrels; but, using the light spherical bullet only, and eight of more drams of powder, + + + + + + + and the velocity and penetrtion at the short ranges at which large game is almost invariably shot will be more than sufficient to penetrate and kill even the largest elephant, whether head or side shot be taken. the accuracy of the large bore rifles and ball guns is very good up to 60 yards with the smooth and 120 with the rifle. The Author (Greener) has in his possession the skull of a large elephant shot by Mr. Carter (smoothbore) of Madras, which shows that the ball from a similar 8 - calibre(bore)rifle(smoothbore) of the author's manufacture--passed right through the skull fro the right to left side, the wound increasing in magnitude as the bullet flattened." 10 drams was the charge, equal to 273 gr. of good grade BP.
: Note that while I have written quite a bit on this subject, I have no first hand experience shooting an 8 bore. My only epxerience is shooting several 12's rifled and smooth, with balls and conicals. The 12 is capable, while the 8 is most impressive, now, as it ws back in 1911, when Greener published the 9th edition.