A few years ago after doing nothing but shoot the heck out of it and putting a couple thousand rounds through my Sav. 93/17 HMR I'd show it some love and give it a good cleaning. I'd already bought a bore guide specifically for the Sav. 93/17 and it's a good thing I did because on the first pass down the bore with a wet patch, I could BARELY get a wet patch through it and I'm not sure I could have had I not had a bore guide because those 17 cal cleaning rods are anything but rigid and this thing was dirty. So after quite some time of scrubbing and cleaning I finally get the bore clean. There really wasn't much copper but it sure did have a butt ton of powder fouling build up.
After getting the bore clean, considering how dirty it was, I'm now thinking if this thing shot well before, boy howdy, this ole girl is really gonna shoot now. Wrong! First trip out with a clean bore and it was shooting groups that looked like you'd just shot the target with a shotgun. It took well over a box of ammo before it started to settle down and go back to shooting like it was before I cleaned it.
I've come to think these factory rimfire barrels are some finicky little buggers and know two are ever alike because some shoot better dirty while others shoot best with regular cleaning then they are some that just don't seem to care and shoot well all the time. Or, unfortunately, there are those that just never shoot worth a darn no matter what you do to them and about the only thing they're good for is an extra long jack handle.
As some have alluded to, don't rule out trying different ammo. Even if it's the exact same ammo but from a different Lot # it'll sometimes shoot drastically different. You wouldn't think the ammo in these little things could vary that much from lot# to lot# but it sure can. I pretty much only ever shoot the Hornady labeled 17gr Vmax ammo because it's easy to get and is generally the cheapest and you sure wouldn't think this stuff could/would vary as much as it does but I've had certain Lot #'s that shot crazy good but I've also had some that shot literally inches different in POI at 100 yards. It's one of the things I hate about rimfires.