Clean the gun with a copper solvent and make sure it is clean. I'm not talking about looking down the barrel (put a flashlight in the open action) and saying "it looks good to me" clean. I mean pushing patch after patch of copper solvent cleaner through it until it comes out as clean as it went in.

If it was my gun and I bought it used, I would then push some JB bore paste (Brownell's)through it and make sure it was clean. This will also remove some of the copper catching burrs in the barrel that exist in just about every factoryinstalled gun barrel out there.

Take a bronze bore brush one or 2 sizes smaller than the bore of the gun you are cleaning.
Wrap it with a clean piece of cotton cloth.
mix the J-B paste in the can (it will sometimes settle) and rub a liberal amount on the cloth.
Run it through the bore for 20 in and 20 out strokes.
Now take a clean patch, soak with HOPPES or similar bore solvent and wipe dry with a couple more patches
Then do it one more time with a new patch on the brush with the J-B paste again.
Be careful not to drip all this crap in the action.

Wipe the gun clean again with hoppes and dry thoruoghly.
I will bet you will shoot the best group with the rifle you ever have.