Here’s my take on this discussion for what it’s worth.
I’ve shot muzzleloaders for over 40 years now, not long compared too some here but here’s what I’ve found.
I’m not a Scientist or an Engineer , but have gained this knowledge over my journey.
Back in the day, (1970’s)
Pyrodex ,Bore Butter & Maxi- Ball’s or Buffalo Bullets coated with a yellow,hard,waxy type of bore butter was the normal combination I used .
I shot whatever load worked best in my particular rifle, mostly 60 to 90 grns charges. All were cap guns .50 cal or less.
Using the old bath tub technique, with Hot water & dish detergent. I trashed more than one barrel!
Pyrodex seemed too cook the bore butter creating a hard carbon like crust in the bore that was almost impossible too remove.
One other variable I haven’t seen mentioned , is the difference in barrel materials now vs then, modern steels I think tend to rust slower..... IMHO
I also think manufactured modern black Powder maybe less corrosive than it’s homemade chicken litter counter part. IDK just MHO.
I never foul a gun before loading, nor do I add any additional protectant too the bore after loading.
Right or wrong , I’ve had no rust issues in any of my gun since I started using Sheath, now Barricade.
I will leave my gun loaded during hunting Season up to one month using a slightly damp patch using Olive Oil as a patch lube.
Rarely have I expierenced a miss fire from my Flintlocks, never have I had too pull the load, they always fired at some point after a little tinkering.
I was also taught too always remove any “oils” from the bore prior too firing the gun, again right or wrong.... IDK, but denatured alcohol works for me.
As far as cleaning the gun, the length of time I take depends on the situation , if I’m putting the gun away for over a week, then it may take me two hours per gun start to finish.
Can I do it faster, sure but I enjoy the cleaning and always break down the lock and do a through cleaning & lubrication prior too reinstalling if it’s not going to be shot again anytime soon.
The two flintlocks I tend to shoot the most are both “in the white”, which I would think would tend to rust quicker than a barrel that has blued or browned finish, browned being a form of rust anyway.
Using Barricade has produced a grayish patina on my guns, which I 0000 steel wool the barrel as needed to control discoloration of the metal, I use a brass brush too clean the pan, lock plate and hammer .
Again right or wrong,IDK but it works for me, and I like the look of the patina the guns are getting.
The one disadvantage I see with the patina , is in certain light conditions it’s hard too clearly see the sights, funny but I don’t recall having those issues 40+ years ago! LOL
As far as brass and BP corrosion goes, I haven’t expierienced any. Discoloration yes, rust no.
This is on solid brass hardware, not brass plated.
With plated brass over time you might see an issue .. IDK.
Others mileage may vary.....