How much of that 12 hours was spent with a bronze brush? It sounds like the barrel was terribly neglected, with layer upon layer of fouling built up. Chemicals work, but there are times when the process is speeded up dramatically by using a tight fitting brush. For the 7mm, use a .30-caliber brush. Use a bore/chamber guide. Use full strokes, pushing the brush completely out of the bore, rotating it one quarter turn then pulling it back all the way through. Use a solvent like Shooter's Choice, don't use the brush dry. Don't use a stainless brush. Buy two brushes and when one gets worn or compressed toss it.
The sharp brushs will tear out a lot of the coarser fouling the fine abrasives just polish - without damaging the bore. The chemicals only work if they can get to the fouling - heavy powder fouling can keep the ammonia from touching the copper. While some guys will warn against using a brush due to "wear" on the bore, I submitt that 12 hours of scrubbing will cause wear no matter what you do - particularly if no bore guide was used.
After a good brushing (ca. 12-18 double strokes), return to tight fitting patches with a good solvent. See if this improves your results. You may want to repeat the brushing. Sometimes a more aggresive approach saves time with less risk of damage.
To better see the extent of copper fouling, put a clean white patch on a jag and insert from the chamber to within 1/2" of the muzzle. Shine a light into the bore, and the reflection from the patch will light up the bore surface nicely. Use a low power magnifying glass if you have one. This is a great way to spot copper or lead streaks, or to see pitting near the muzzle.
