12-15? Yikes! I once had a really rough military barrel on a Garand that started to fall apart in the middle of rounds 30-50 of the National Match Course (600 yard slow fire), and have heard of guns doing it after 20 rounds, but yours takes some kind of prize. A lot of barrels need ten rounds to really settle, so you are getting like zero good round time.
In your shoes I would do a couple of things. First, I would make sure I got the barrel truly clean so that I knew I wasn't starting out pre-fouled. No way to know how the previous owner treated it and old fouling can be a misery to get out.
The method I would use is to start with some Mobil 1 0W-20, and get the bore good and wet with it and let it sit 24 hours and then push a bronze brush from breech to muzzle, unscrew it at the muzzle, remove the rod from the bore, put the brush back on and repeat a couple or three more times. This is so you are only brushing in one direction. Then patch the oil out. Mobil 1 has very good carbon softening additives. The Army Marksmanship Unit uses it for this reason. If you have hard carbon deposits, the patches will come out blackened with visible carbon (not just gray steaks). If you get that, I would repeat this every day until black carbon stopped coming out.
Another excellent product for that is Gunzilla. It is actually a little faster working, IME, but oil is cheap if you have time to let it work. Gunzilla I have left in a bore for a month and a half while it kept getting carbon out of pits in an old Springfield barrel, so it's something to try if you are for some reason not satisfied with the oil results, but I expect you will be.
Next, I would spray some brake cleaning solvent down the tube to remove the oil. Brake cleaner, unlike carburetor cleaners, has no residual lube left behind.
The next thing I would apply is KG-12 copper solvent. This is not like the usual ammonia copper removers. It is a water-based chelating chemistry. If you let it sit 20 minutes it will absorb a huge amount of copper. The only drawback is it doesn't turn blue or green when it reacts with copper. It just gets darker orange. But three applications over an hour will remove more copper than I've yet found in a barrel.
At that point, if you know someone with a borescope, I would take a look, particularly at the throat and first two or three inches of the bore, where copper fouling is heaviest. If you don't have access to a scope, I would apply a copper solvent that does turn blue, let it sit five minutes and push it out with a plastic or other non-reactive jag, just to be sure I didn't see any color coming out.
At this point I would take
Iosso Bore Cleaner or
JB Bore Compound and push a patch down to the muzzle, remove it, push it through from the breech again, and go for a third or even a fourth time if it still seemed to be working. I would then get a fresh patch and repeat, and keep repeating every third or fourth patch until I had pushed through the bore the bore a total of about 50 times (48 is divisible by both three and four; close enough). Then I would repeat with Flitz.
Now take the gun to the range and see if you haven't made progress with the fouling problem. If not, you may need to look at firelapping or other more aggressive polishing methods. I don't like it as a first resort, because you very occasionally get a gun that sees adverse accuracy effects from doing it.
myth further busted .