Overall that barrel had ~250 rounds when that photo was taken. You'll have to excuse my poor practice up to this point, but that barrel is usually "cleaned" after each trip. Probably 20-40 rounds between cleaning. I had not at that time begun my in-depth cleaning regime targeting the throat/free-bore and leade. I only noticed the issue with the carbon ring when the bolt close became heavy or impossible, and the ogive was showing scratches after unchambering a round. I include all this to say, what I thought was good was not in fact even close.
When I started tackling the carbon ring, I took an overall approach and used Eliminator and C4 throughout the whole bore to clean everything out. It wasn't perfect when scoped, but the majority of carbon and nearly all of the copper was removed. But still that didn't even put a dent to the carbon buildup in the throat. So my target approach went like so:
First, I got a nylon brush the next size up and applied to C4 to target the throat. I scrubbed in circles maybe 50 rotations but that didn't really touch the carbon. More of a tickle.
Next, I took the same oversized nylon brush and wrapped a 2" square patch around it, soaked with C4, and stuck it into the throat and let it sit. I set a stop-watch and gave it a good 10-20 rotations every 15-30 minutes. Repeat for ~6 hours. I wasn't really watching the clock too closely, but after 6 hours I pulled it out and the patch was absolutely black with carbon.
I did another round of scrubbing the bore with a new 6.5mm bronze brush soaked with C4 and a few dry patches in between. Maybe 3x30 strokes with the brush. Though this didn't really touch the throat. So I decided to repeat the soaking process for another ~4 hours. After that was done, I saw what was posted above, a completely bare throat. And the leade was impressively shiny clean as well.
Otherwise, I didn't run any patches through the bore aside from what I used combined with the brushing at the 6 hour mark.
A bit off topic, but after completely cleaning the throat, I went and shot 25 rounds of much newer Hornady 140gr ELD-M (Lot 3246484) and it still managed to utterly carbon foul the throat. I read around about that generation of 12LRP had issues with inadequately reamed chambers, which might have been the case. But for about the same price as getting that cluster unfucked I opted for a new Shilen bull barrel and so far it has been absolutely fantastic. Also, to their credit the Savage barrel change process is very approachable for an amateur.
