I am not an advocate of leaving a rifle fouled for extended periods of time, but with some powder, the rusting clock sure seems to slow down. BlackHorn 209 I have read is one such powder. Another is Triple Seven. Some people leave their barrels fouled for up to a week. Ask Sabotloader how long he has gone between cleanings with Triple Seven.

I would never try that leaving them fouled with Pyrodex or Black Powder. Although a few years back I did a test with Pyrodex, APP, Triple Seven, and Goex I believe was all of them. And I cleaned barrel steel then placed powder on that barrel and ignited it. Then just let it sit. The control sample being clean steel never rusted of course, but Pyrodex RS and Goex did rust with in a couple days.