I own 4 sidelocks and found this out just last week. I use bore butter and will continue after I found this out. Last weekend, I shot my .50 t/c hawken flinter, .50 t/c hawken percussion, and .54 t/c renegade percussion. The flinter I've had for years, the hawken percussion I only shot a couple of times, and the .54 renegade was brand new and cleaned with a solution of 50/50 windex and alcohol to get rid of any oils.
After the shooting session I put some hot soapy water in a bucket, brushed and swabbed all the barrels then rinsed with hot clean water.
I then preceeded to run patches thru the barrels - both dry patches and ones soaked with #13 BP solvent.
The flinter barrel patches came out clean after only a few swabs. The hawken perc. took a few more patches to clean and the renegade was a mess.
IMO the bore butter does 'season' the barrel and inhibits rust and makes cleaning much faster.