Well, there you go. I used to always say BH can rust a barrel in certain conditions, but everybody had me convinced it couldn't happen.

I'll go back to saying it can happen on some guns.

It certainly can happen even to SS barrels. You just have more time with SS. Pyrodex is plain terrible as far as rust goes. I would rather use real BP than Pyrodex.

If you want to use BH209 over an extended period on a fouled bore, dry swab to remove most of the fouling. Extended to me is a 3 day weekend without any rain.

Yes, i agree, one pass with a dry patch, should greatly help reduce the chance of rust when using Blackhorn. After one pass with a dry patch, my rifle barrels look shiny clean, even though i know they aren't. Using a patch with solvent proves the barrel wasn't clean.

The use of one dry patch removes the bulk of the loose burnt stuff, and theoretically should put off rust for quite awhile. It may also help with the first shot flier.

Myself, I have always followed a certain cleaning ritual that my dad and greatgrandpa drilled into me, "If it left the house, it's to be broken down and cleaned immediately". That goes for ALL my weapons. Even my knives. It's partly why I did that cold/clean barrel test. It's been a rare thing over the years that I have ever had to take a followup shot with a ML'er but it HAS happened. 1rst shot performance is what is key in ML'ing. Game doesn't have the tendency to "stick around" while you load up a second. If I had your rifle, I would set it up for where that first shot is hitting with whatever you decide on to shoot. Shooting groups is all well and fine for the range to see what your rifles overall potential is but it's that 1rst shot that counts on game.

Your gonna like those 300gr XTP's in a black crushrib!!

So what are you using for powder these days? You don't like BH209, triple seven, or pyrodex...

I wonder if dad-in-law was exposing his already shot ML to a mix of high indoor and low outdoor temps, allowing that gun to sweat for multiple days / times? That probably increased his odds for rusting. Moisture may be the reason the non-treated breechplug threads and edges froze-up.

I never had a breechplug even come relatively-close to freezing-up using Blackhorn. I use the white plumbers teflon tape on mine. I give all threads two layers of protection. Then magic appears when I clean that breechplug. I would say 80% of that teflon tape comes up missing........ poof..... gone..... MIA.... like in a Randy Wakeman Magic Show in a Chicago 150-seat nightclub.

Blued guns are terrible with any powder.

I keep the BP threads bone dry when using BH. It spins right out everytime. They'll tell on their site that nothing is needed on the threads.

I'll keep track of the 1st shots for that approach. I will also try the dry patch between shots. Dirty barrel seems risky, I will not know the weather. This is a high altitude horse hunt.

Then you will DEFINITELY NOT want to have any fouling in your barrel period. Not sure where you will be but every altitude hunt I have been on in my considerable years has been like a woman when it comes to weather. Never know when it's gonna change and never know if it's going to be a storm or just a mild fit Not to mention altitude change means temperature change as well as moisture density. That equals condensation in the barrel. Stick with the plan, clean your bore down well for EVERY shot, cool it, and basically shoot it at the range every shot as a cold clean bore. Figure where that shot is hitting at consistently.