The "Thunder-barrel" mentioned in the O.R. by Lt.'s Synder and Seymour are detailed by Chester as follows.
The "flying fougasse," or bursting barrel, a device of Captain Truman Seymour, consisted of an ordinary cask or barrel filled with broken stones, and having in its center a canister of powder, sufficient to burst the barrel and scatter its contents with considerable force. A fuse connected the powder in the canister with a friction primer in the bung, and the barrel was exploded by attaching a lanyard to the eye of the primer, and letting the barrel roll over the parapet, as in the case of the shell-grenade. If one experiment can justify an opinion, the flying fougasse would have been a success. When it became known in the fort that one of the barrels was about to fired as an experiment...the little crowd attracted the attention of the enemy...When everything was ready the barrel was allowed to roll over the parapet, and an instant afterward a terrific explosion took place. The stones were thrown in every direction, and the surface of the water was lashed into foam for a considerable distance. The effect as seen by the secessionist must have appeared greater than it did to us, although we thought it quite satisfactory...Only three of them were constructed, yet for moral effect an empty barrel set upon the parapet would have been just as good.
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More from Vicksburg...
[HDQRS. DEPT. MISSISSIPPI AND EASTERN LOUISIANA,
Vicksburg, June 4, 1863.
Maj. Gen. MARTIN L. SMITH, Commanding Division:
GENERAL:I am directed by the lieutenant-general commanding to
say that you will have prepared by your ordnance officer, as soon as
possible, a number, say four dozen,” thunder-barrels,” petards, &c., for
defense of weak points on the line. It is desirable that materials for
this purpose shall be as far as possible such as cannot be used with the
guns now in position on the line. If materials for the above are not
in your possession, you are authorized to make requisition on the depot
ordnance officer.
I am, respectfully, your obedient servant,
R. W. MEMMINGER,]
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I would conclude the thunder-barrel, which is mentioned by name at least four times in the O.R. documentation for the siege of Vicksburg and once for the federal defense of Fort Sumter at Charleston, and by the name "flying fougasse," or "bursting barrel" by Captain Chester was an improvised device but still a formal military device taught to military engineers. As mentioned in the Vicksburg reference above, no description of the device was required when Pemberton ordered the construction of nearly four dozen of these devices by General Smith's ordnance officer.
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David Upton