TL;DR: I have done my research and lots of tinkering here, and I'm hoping to get confirmation/refutation of my working theories on how to solve my 9mm leading problem once and for all. Also need help finding a good lead solvent so I can get my barrel back to "like new" condition.
I have been casting for my 9mm for about 6 months or so now, with good success. I have tried a few different powders, lubes, etc. Most recently I've been shooting Lee 124gr RN TLs powder coated on top of 3.9 grains of Bullseye. It's a good load and quite accurate.
Unfortunately, ever since I switched from W231 to Bullseye, I've been getting some leading, particularly in the muzzle end of the gun. Lyman's cast bullet handbook says leading only in the muzzle is often an indicator of "running out of lube." I don't think that's the case since I'm powder coating instead of using 45/45/10 these days.
After lots and lots and lots of reading, I've come up with and tested various working theories of what's contributing to my leading:
1. First guess was that my cases were swaging my lead down, either due to improper sizing, not enough mouth bell, or too much taper crimp. My barrel slugged at .357, so I size to .358 after PCing. I purchased a bullet puller and checked several bullets from mixed brass after my normal belling, seating, crimping routine. I bell liberally and crimp conservatively. Turns out I was not getting any swaging, so I ruled this out.
2. Next guess is that my alloy is too hard to fully obturate, leading to gas cutting. My bullets are all cast from 100% mixed clip-on and stick-on wheel weights. I don't have a good source of pure lead to alloy them with for that ideal 50/50 mixture. I don't have a hardness test available (or $60 to blow on one right now), but I'm guessing my air-dropped bullets are probably somewhere in the neighborhood of 15 BHN after they have hardened a week or two. If someone else has a more informed estimate, I'm all ears. I'm thinking I may be able to help fix this by sizing to .359 so less obturation is required to get a good bore seal.
3. I have been guilty of letting friends shoot j-word factory ammo through my gun at the range, in between my normal cast handloads. I'm reading now that any amount of copper fouling in the bore can act as a "seed" for leading to really stick. I've been very good about cleaning my gun using Hoppes #9 and CLP in between range trips, but I also think I've left bits of leading in the muzzle that have also acted as a start point for future leading on the next trip. So this leading is the gift that keeps on giving--even if I solve all the other factors causing this, I need to get my bore CLEAN and back to shiny new or I'll just keep riding this lead train into the station.
4. Examining my notes on leading while I was developing loads with both W231 and Bullseye, I noticed that my go-to load of W231 (4.0 grains) did not lead my bore. When I decided to try out Bullseye hoping for better accuracy, that's when my leading problems began. Bullseye burns significantly faster than W231, and I think that's causing higher pressures and making the gas cutting and bullet-base-melting a lot worse. Luckily, I can't get Bullseye to shoot as accurately through my M&P as W231 anyway, so I'm switching back. The slower-burning W231 should hopefully help me control leading.
So I need to get my bore spotless. Last night when I got back from the range, I hit the bore with a soaked Hoppes #9 patch, let it sit for 15 minutes, and scrubbed it for a good 20 minutes with a worn out bronze brush wrapped in copper Chore Boy. I made some progress, but there are like 2 grooves in the muzzle that just will not come clean. They have some really ironed-on-looking leading that I can't budge with any amount of solvent or elbow grease. I gave it another 30 minute soak in Hoppes #9, scrubbed again for 20 minutes, and made almost no visible progress. I ended up soaking in Hoppes #9 overnight and scrubbed again this morning. Still barely any progress. I had to leave for work, so I soaked it in CLP before leaving since the Hoppes #9 doesn't seem to be making much of a dent. When I get home I'll scrub again, but I'm thinking I need to pick up a good lead solvent.
I've done some reading and seen proposed magical lead-fixing solutions ranging from "didn't work" (i.e. Hoppes #9) to "will 100% destroy my barrel" (i.e. soaking in hydrogen peroxide + acetic acid overnight).
Is there a good lead solvent that I can use to get out these last stubborn vestiges in my muzzle? Preferably something that won't mess up my bluing and stuff, but I'm taking all recommendations under consideration at this point.