Question on caps/load:
1.) I know the black powder is very corrosive and I've taken care to always make sure I clean the barrel thoroughly with soap and water after firing my ML.
However, How corrosive is the residue from the caps?

let me expand: before hunting, I always like to fire off a cap to ensure the bolster is clean and see a good burn pattern on a patch. Then I load up and hope for luck... Currently my season has been bleak and I haven't fired my ML since I loaded it 3 weeks ago and I'm wondering if you think this is wise... Do you always "unload" your ML after hunting, or keep it loaded 'til next time?

2.) Has anyone found that keeping powder/patch/bullet in your ML for an extended period of time (i.e. days/weeks/months?) before firing it alters the performance of shot when it's finally fired? (i.e. more hangfires/less accuracy/etc...)

I found out the hard way that remington#11 caps are corrosive . Thankfully the pitting from them is only around the nipple area and only affects the looks, but it was a real bummer. I was always told the residue was not corrosive , I have a beautiful double barrel 58 cal that says otherwise.

As Sharpshooter so stated... any thing you fire, is technically corrosive. If you fire off caps, and then don't clean the area.. you probably will over time find the start of corrosive forces working. The rust is normally not too wide spread from caps.

Leaving a rifle loaded for extended periods of time. All depending on the environment you hunt and live, whether it will effect the accuracy and over all function of the load itself. In Northern Wisconsin where I live its cold, wet and humid. Not a good combination for rifles, powder, etc. When I hunt in snow, I always unload at the end of the day, clean the rifle, and start new the next day. But I am sure there are others that do not. Its all a matter of preference.

I have had misfires from leaving the rifle loaded, and even hang fires or half fires. So again, do what you think is best. IMO and only my opinion, 3 weeks is too long to leave a rifle loaded and unfired. Were it me, I would answer a lot of questions for myself, next time in the woods. I'd pick out a spot on a stump, get a good rest and let fly. I'd see if it hits where it is aimed, and then swab the rifle clean. After that I would check it for all the rust and fouling possibilities that you currently question. Knowing what I can expect is a great satisfaction.

thanks for replies.
following up on cayugad's comments: (and I live/hunt in north/central MN so our climates are similar)
Do you have a way to check the inside of the bolster for rust/fouling?
(For my barrel, I usually identify fouling/rust on the basis of the appearance of the patches when swabbing... do you do more?)

Unfortunately inside the bolster is hard to check. If you pour solvent down the barrel you could watch and see what color the solvent comes out as. But I really know no way to check the inside of the bolster. Even if you take out the bolster clean out screw and insert a tooth pick to see what comes out. Since we are talking side lock, I would definitely not leave it loaded for extended periods of time. I did this with my Renegade one year.. And finally I was on stand and in walked the first legal buck (very small spike) that I had seen in over 10 days. I took my bead, the cap went pop.. and no boom. I even got a second cap on the nipple and again.. pop. Well the buck went bye bye. I finally had to pull the nipple and drizzle powder down the nipple port, using a nipple pick I worked as much powder in there as I could, and put the nipple back on. The next cap fired it. At a tree stump. And I loaded immediately and tried it again, and it fired. So I am guessing the powder was compromised somehow during the storage. After that I stopped leaving them loaded.

I would fire the rifle off.. then use a hot soap and water bath to really flush that bolster. And after that pour some alcohol down the barrel. And let that drizzle out the bolster. Then you could oil that up and set it up right, any spare oil will work down there.

At least a bolster can be replaced.

The priming compound in today's percussion caps is not corrosive like the old corrosive primers that used potassium chlorate as an oxidizer. Those left a corrosive residue of potassium chloride salt in the bore.

However, the residue left by modern #11 caps can hold moisture on the metal in a humid environment. So even though the residue itself is not a corrosive compound the moisture encourages rust.