If you are talking about black powder cartridge rifles, yes they are very accurate.
I shoot BPCR silohette in the summer. The sharps, hiwalls and rollingblocks are very accurate rifles. the rams are at 547 and are usually fairly easy to hit. We have a ram hanging at 647 up in the woods on a hillside and go shoot it quite a bit. if you are a decent shooter it is fairly easy to hit it time after time. I have hit a 2x3 target at 925. so yes blackpowder is accurate. Read, shooting old buffalo rifles by Mike Venturino if you are interested in these type of rifles. Mike is an authority on these rifles and is also a very nice gentleman and a tough fellow to beat in the matches.