According to the author Joseph W. McKinney, in his book ‘Trevilian Station’, Brigadier General Wilson wrote to the Chief of Ordnance stating that ‘the cartridges were impervious to water, they could be reliably loaded and fired in driving rain or immediately after being immersed in a river’. The author finishes with the sentence, ‘soldiers need not worry about keeping their powder dry.
Along a similar line, Roy Marcot, in his book "Spencer Repeating Firearms", at p. 79, reprints in full the February 18, 1965 "Memoranda Report of Arms" made by William N. Gamble, Col. of the 8th Illinois Cavalry and commander of the 1st Brigade of the 1st Division of the Cavalry Corps of the Army of the Potomac. Gamble says the Spencer cartridges are "not affected by dampness". He then gives a fuller description under the heading "Ammunition":
"AMMUNITION
It would be an immense saving to the Government, and greatly increase the effectiveness of troops in the field, if metallic percussion ammunition could be used with all breech-loading arms. My knowledge of this is wholly practical, having been in the field nearly all the time since this wa[r] began. I could not urge this matter too strongly, because I have seen its necessity so often in many a warm place!
Metallic ammunition warrants a shot every time, without caps; is not injured by dampness and can be carried in a cartridge box on horseback for a year without being injured, while two-thirds of the other cartridges are actually a clean dead loss, either fall to pieces, or get injured by dampness."
Gamble also includes among others in his report the following assessments from the 5th Division and from the 4th U.S. Cavalry:
"Remarks -- 5th Division
We left White's Station, Tennessee on September 30, 1864 and marched 400 miles. The weather had been very wet for 4 or 5 days and the ammunition was carried mostly on pack mules. Upon inspection, it was found that the Spencer ammunition was not injured by the weather, or the wear from carrying in saddlebags. 75,000 rounds of Sharps and 12,000 rounds of Colt's rifle were spoilt or broken up, so as to be entirely worthless.
Remarks -- 4th U.S. Cavalry
Two boxes of Spencer ammunition (1008 rounds each) were sunk in the Duck River for nearly two days, and the ammunition upon being taken out was only partially injured. It is seldom that a Spencer round ever misfires.
Respecting the merits of the different arms, there can be no two opinions concerning the superiority of the Spencer repeating carbine. In the hands of a reliable man, it is fully adequate to perform the work of 4 ordinary breechloading carbines. Rainy weather is no deterrent to its efficiency."
Marcot found the report in Nat'l Archives Record Group 156, Entry 201: Record of Experiments.