Shooting a lot doesn't just mean powder fouling in the bore. Shooting cast bullets can leave lead fouling, and jacketed bullets-especially in rifles with their higher velocities- will leave copper fouling behind. If a bore is a touch rough, only a few shots can cause a problem, while a well-finished, polished bore may take a lot to become fouled.
Now comes the real fun part, getting that crud out. There are lots of different solvents, from the old Hoppe's No. 9 to some very fancy new stuff. Lead being relatively soft, it can be a lot easier to get out. My favorite cleaner for it is called Blue Wonder. It's a thin gel you use with a bronze brush, and it works.
Copper is a different matter. It's a lot harder than lead, and it's a pain to remove. Most of the really good cleaning solvents for it stink because they contain ammonia(which attacks copper). They work, some of them quite well, but they can involve a lot of scrubbing. There is, however, an easier way.
The easier way is Outer's Foul Out. It's an electroplating system. You degrease the bore, then put a rubber plug into the breech to seal it. There's a steel rod that you slide some o-rings on to insulate it from the barrel and slide in. Then you fill up the bore with the cleaning solution- one for copper, one for lead-, hook up the unit and turn it on. It actually pulls the fouling off the bore and plates it onto the rod. Lead comes off pretty quickly and just wipes off the rod; copper takes a lot longer, and has to be sanded off the rod when done. The nice things about it are: a. it actually does work, b. you can set it up and do something else for a while as it works, and c. it doesn't wear on the rifling as lots of scrubbing can. It ain't perfect, but it can really help.
One piece the Foul Out really helped on was a SIG Trailside pistol. It's a .22 semi-auto made for target shooting, and wonderfully accurate. This one had a bad problem, in that by the time you'd fired 100 rounds the bore was so badly found accuracy was lost. For a couple of reasons I didn't like the idea of scrubbing this bore with solvent and a brush to get it all out, so I set up the Foul Out and let it work. It cleaned the bore down to bare steel. Side note: I then treated the thing with Microlon Gun Juice, which took care of the problem, no fouling problem since.
There's a lot of cleaning stuff out there. Brownells now carries a stuff called Copper Melt that's supposed to work very well, I haven't tried it myself(at about $30 a bottle, it'll be a while). For the bad copper for now, I'll keep using the Foul Out.