Here's an illustration from the Union's ORs, for June 30 1862, assuming the Trent shutdown alluded to continued but usage did not alter.
Remaining in stock OTL on 30 June 1862 (Ordnance dept.):
1,036,871 lbs powder
9,054,435 lbs saltpetre (incl. army reserve of 3.8 million lbs, which is not very high quality as it's been stored for about 15 years)
(combined this is the equivalent of about 13,078,000 lbs powder in store, a little under twice the expenditure since the beginning of the rebellion)
Imported from Britain (
summary of British trade) and shortfalls
DuPont purchase of saltpetre 5.2 million lbs (about 6.91 million lbs powder equivalent). 31+172+323+50+49+10+15+25+25 = 700 tons also imported from Liverpool, which clearly is not the DuPont purchase as the DuPont purchase is well over 2,000 tons and was not spaced out over more than six months.
Average yearly imports of salteptre from India "direct to America" is about 8,000 tons saltpetre per year over three years (assuming a constant rate, this is about 4,000 tons saltpetre or 11.7 million lbs powder equivalent in the first half of 1862)
Saltpetre shortfall approx. 5.5-7.5 million lbs powder equivalent. Even assuming that the rate of imports is abnormally low in H1 1862 the Union is running on empty for powder by the end of H1 1862, which means fewer rifle rounds per man and less ammunition per fort/ship/battery. It also means that the powder quality will dip markedly some time in April/May as the army reserve is tapped.
OR Series III Volume II pages 852-853 states that at this point the Union had 1,911 tons on hand.
Now, since Series III Volume II is dealing with the
post Trent period, it should be pretty obvious that the fact they imported 2,300 tons of saltpetre from Britain in the DuPont purchase (which was stopped by the Trent) would cause a problem if in Series III they described themselves as having 1,900 tons of saltpetre in reserve. To put it bluntly, remove the DuPont purchase alone and the Union's 1,911 ton surplus becomes negative four hundred tons - the Union has run out of powder.