I don't know if the following helps or confuses the matter. I found it in the ORs Series 4, Vol 1, Part 1 pg 407.

Augusta Arsenal, Ga, June 24, 1861

Major J. Gorgas,
Chief of Ordnance, C.S. Army, Richmond, VA.:

SIR: I have the honor to return the letter of Governor Brown to the Honorable Secretary of War, and in compliance with the indorsement upon it to report that upon a careful examination of the contents of our magazine I find on hand 394 barrels Du Pont cannon powder, 1860, 9 barrels old, and 400 1/2 barrels Hazard powder, making of the Du Pont and Hazard powder 49,000 pounds. Deduct the 200 barrels received a short time since by me from Memphis, and we have 29,400 pounds of cannon powder, which is probably the powder claimed by Governor Brown. I find upon inquiry of Mr. Butt that 10,000 pounds of powder were shipped from our magazine during its occupancy by the State of Charleston and Savanah. This would make 39,400 pounds, or 400 pounds more than claimed by His Excellency. I have but 9 barrels musket or rifle powder of Du Pont's or Hazard's make on hand of recent date. I have 20 barrels of 1838 and some of Garesche, 1839. This powder, though, was that originally owned by the United States. We have on hand 24 boxes of 12-pounder canister shot, fixed, and 3 boxes of 6-pounder shot, fixed. Shall I not send them to you?

Very respectfully, I am, sir, your most obedient servant,

F.C. Humphreys,
Military Store-Keeper of ordnanace, C.S. Army

comment: How long was stored powered go for? Some of this stuff Humphreys is citing is 23 years old! Its tempting to just add up the number of barrels cited and dividing it into the total number of pounds. Unfortunately, I've found another reference suggesting that Army rifle powder (fine grain) is twice as dense per unit volume as Navy cannon powder (coarse grained). I'm assuming that Army and Navy cannon power must both be coarse grained or the Army charges must be half those of the Navy.