The reason you do not remove the pinned barrels is as already described. The stock to barrel fit gets loose over time and the accuracy suffers.
A friend I hunt with has the same rifle. That is one tack driving rifle and it does not seem to care if he is shooting 30 grains or 100 grains. I know first hand it will drop a deer with a roundball in its track (other then the deer jumped six feet straight into the air and then crashed back down to the ground when he shot) at just over 100 yards.
My friendtook a second nipple, drilled it out, then took plastic fish tank hose and epoxy glued it to the nipple. On the other end of the hose he out a large lead sinker and glued the hose to that. He just puts the replacement nipple in the rifle with the hose attached, drops the sinker into a bucket of boiling hot soap and water and and pumps the hot water through the bore until the rifle is clean. Then he bore brushes the barrel with solvent and repeats the bath. After that, he dumps the dirty water and pours boiling hot water through the rifle to clean out the soap, which catches in his galvanized bucket..
After that he does the bore butter smearing.. and has not complained of a single problem with it's use.