The powder will not degrade any faster in the cylinder than it does in the factory container. I am very meticulous in cleaning my revolvers as well as loading them. I keep 2 loaded in the house all the time. I usually fire, clean and reload them at least every 6 months or so. I have gone longer at times. Been doing this for over 30 years now, and I can honestly say I cannot remember a misfire on the initial charges at the range. I have had the typical problems (not often though) of a cap dropping in the action after firing. But a failure to fire due to powder - never.
Having said that, I will add my guns are pretty much kept in ideal conditions.
Exposure of the powder to dampness would be my only concern, more so prior to loading (that will be obvious). Your powder is not going to fail unless it is exposed to moisture.
I dug a can of powder out of a locker that I had forgotten about, been close to a year ago now. It had to date back to the early 90's. Still shot good.
I also shoot black powder cartridges. I have some of those that are several years old and they are as reliable as any smokeless cartridge.
The residue after firing is what is really corrosive. Never have had a problem with the unfired powder.
Short of fording creeks or going swimming with your gun, you most likely will be wanting to go to the range long before the load componets go bad.
It depends on how clean and dry the cylinder was when the charge was loaded, how tight the cap is in place, how tight the seal is around the ball and the humidity level. I would not count on more than a week or two around these parts.
Tom Kelly posted a great article on this subject in 2000. Hope this helps
How Old is Too Old?
Were you ever rummaging around in the back of your shooting safe, closet or gun room and you came across a box or two of loads from the past? What was the first thing that came to your mind? "Are these still any good?", wasn't it? I think you'll be surprised at what I have found out in the course of a two-year study on the subject.
There are as many old wives tales about the shelf life of blackpowder as there are old wives, so it was hard to separate the wheat from the chaff in developing this article. Some skirmishers say if it's older than a couple months, don't use it. Others point out that antique cartridges like .44 Russians and .45 Colts can still be fired 100 years after they were manufactured, which is true. Just what is the story with the shelf life of blackpowder loads; how old is too old?
In order to develop the data for this article, I used some .45 Colt loads for my Henry and some loads in my Harpers Ferry M1855 three-bander. The Henry loads were 18 months, 10 months and 2 months old, and were all a 185-grain semi-wadcutter on top of 31 grains of FFF blackpowder in a Remington-Peters case with a CCI 350 primer. I chose this load because, for one thing, these were the oldest loads I had sitting around when I started prepping cases for this study. Additionally, I have used the load successfully in Cowboy Action Shooting events, so I know it is fairly accurate out to 25 yards or so. I decided to run these loads through the chronograph and see how the mathematics of the loads held up, and I was also interested in group size, as a practical application of the data. Believe it or not, I actually had some 10 year old musket loads in the back of the gun room, and I tested those loads against some fresh ones. These old loads had the T&T Minie, which is no longer available but was a swaged bullet that had a sharp point that shoot well at 100 yards and beyond because it didn't have the flat point that cast bullets do. For the test, I dumped half of the found loads out and replaced the powder with fresh FF.
The musket loads were tested in typical Ft. Shenandoah weather, the temperature and humidity were both in the high 80's. The 515 T&T Minie Ball shot good 50 yard groups with both the 10 year old load and the 1 day old load, but the group for the 10 year old load was a little tighter, which surprised me. When I took the chronograph in and downloaded the data, I was surprised that the 10 year old load was hotter than the fresh load!
The table below summarizes the results of the T&T aged powder test. The older load was hotter, but the mathematics of the two loads are strikingly similar past velocity - the spreads and standard deviations are statistically equal. These loads had been stored in cardboard load tubes, not plastic, and it will take a while for me to conduct a 10 year test on plastic tubes. The loads were stored in a cool basement or closet for their storage period.
Load - 39.5 gr FF ----------1 day old-----------10 years old
Lowest Velocity (fps) ---------663.0---------------832.2
Highest Velocity (fps) ---------886.5---------------1062
Average (fps) ----------------748.3----------------886.2
Extreme Spread --------------223.4 ---------------230.6
Standard Deviation -----------73.04----------------71.49
The Henry testing did not consider loads as old as the Musket testing did. For the Henry test, I was really testing the effects of long-term storage on black powder cartridge loads. For this test, all cartridges were identical, only the age was different. The cases were all Remington-Peters 45 Colt cases, CCI 350 magnum primers were used exclusively, and the primers all came from the same 100 unit package for conformity. Likewise, all of the 185 grain bullets came from the same 100 unit package. Even the powder was from the same 5-pound package.
The table below illustrates the results of the cartridge test. All three loads shot about a 4 1/2 inch group at 50 yards, with very similar statistics. While the freshest loads were the slowest, they maintained the best extreme spread and standard deviation results, too. Personally, I don't think a clay pigeon or a wood block much cares if it gets broke by a bullet travelling 1147 feet per second or 1235 feet per second. While I don't favor this light of a bullet for skirmishing work, these were the oldest loads I had when I started the test, and I suspect that I will find similar results in the future. Only problem is, I shoot so much every year I don't have any leftover loads to save for a test!
Load ---------------2 months -------8 months ------18 months
High Velocity (fps) ----1169------------1265 -----------1258
Low Velocity (fps) ----1133------------1222------------1209
Average Velocity (fps)-1147------------1244------------1235
Extreme Spread -------36.47-----------43.35-----------49.42
Standard Deviation ----15.19-----------16.49-----------17.46
In summarizing my results, I would have to say that these two studies demonstrated that properly stored black powder loads can be stored for long periods of time without effecting the accuracy or performance of the load. Now that I know what I always thought I knew, I will have no qualms about making up lots of loads for future skirmishing use. In the past, I would only load up enough for the next two or three shoots, which is usually only a couple months worth. Now, I am starting to suspect that fresh loads may need a week or two of storage to "settle and age." Given the opportunity, I will go ahead and load up enough for as many as a whole year's worth of shooting. As you are reading this in November or December, maybe you might think about making up the loads you will need for next year's shooting while things are slow, the grass doesn't need cutting and the hay is already in the barn. That's what I'll be doing.
Until the next time, promote responsible gun ownership, shoot safe and have fun. Happy Holidays y'all.