Northport buckslayr
You have been introduced to the famous
Underclocked 'Crud Ring'.
Shooting T7 often creates this ring and stops the proper seating of a bullet as the heat of the explosion in the bore combines with other things that might be in your bore and creates a burnt residue ring.
Using a synthetic oil in the bore instead of an petroleum based oil and certainly an oil free of Teflon may help reduce the amount of the ring.
A simple way to get rid of the ring is to run a moist windex patch after each shot... I use the same over and over several times. If you make the patches with the correct amount of moisture you will not even have to run a dry patch... the ammonia in the Windex will dry and carry a certain amount of water with it...
Here is a picture that might help you pre-make some patches... Patching between shots also helps the barrel to cool before shooting the next shot.
The other thing you can do is shoot 'Black Gold' - BH 209 it has a bit of a learning curve but does not cause the fouling crud ring that T7 does.... I am still to cheap so I keep shooting T7.