I just saw this thread so forgive me if I repeat some of the tips already mentioned. As already stated, there is a lot of carbon in that barrel. I used to be in the "don't clean it until your groups open up" crowd but changed my tune last year. For anyone reading this, if that method works for you, by all means continue to use it. I'm going to offer a different cleaning method that works for me. I have a few rifles that shoot great without cleaning but I've had a 28 Nosler for about 8 years that I couldn't develop a load for. I sent it back to the original gunsmith and had 2 others look at it but they all said it looked fine. The last GS said the barrel looked good but it was filthy.
How could it be filthy when I cleaned the heck out of it and the patches came out clean? So I bought a Teslong borescope and looked at the barrel. It was black and cracked for the first 3 inches of the barrel. I had used Hoppes, Sweets 7.62, Boretech, Wipeout, CLR, etc. on this barrel and the patches always came out clean. I ran across a video with Erik Cortina talking about using IOSSO. I've always heard not to use abrasive pastes but I figured I would try it since I was going to rebarrel the rifle anyway. I used it on a bronze brush and it looked like black mud coming out of the barrel. Took the rifle to the range and holy cr@p! What a difference. The rifle went from shooting 3" groups at 100 with horrible ES to shooting .3" at 100. After researching more about cleaning, I have found that IOSSO will not harm your barrel. Neither will bronze brushes, using a drill or reversing mid-stroke. A lot of these statements have been around since I was born and maybe had some merit at some time but not now. Top F Class shooters use some of these methods (minus the reverse stroke).
So, long story short, I use IOSSO on most of my rifles now. How often depends on the rifle and powder used but as a rule, pretty much after every shooting session. I also use bronze brushes
. I have talked a couple of my PRS buddies into using it and it's helped their rifles as well.
I have attached a video of the throat of my 28 Nosler after shooting only 24 rounds. I'm using R33 which is a very dirty powder but you can see the difference in cleaning. Carbon cleaner used was Bore Tech C4. Give IOSSO a try and report back on your results.