Oleg,
What an interesting and unique question. I do not think that a "rock" would be a very good projectile out of gas propellant operated guns like we commonly have at our disposal. If you can find some containing a high percentage of various ores, and then purify it to an acceptable degree (ie.smelting), cast or forge the bullets, it may work. But then, that is what we have today... I think that raw iron would work, for instance, but be very damaging to the bore of your firearm. Its just too hard.
On the other hand, there are a range of slings and sling shots that can mechanically propel stones of different sizes (calibers) and densities with relative accuracy and effectiveness. Catapults were used throughout ancient history to propel large stones at long ranges, as a precurser to the cannon. A movie I saw once showed farm livestock being hurled by a catapult, even a cow. (Monty Python's Holy Grail?). On a smaller scale, I fondly remember the days as a youth going rabbit hunting with my ol "wrist rocket". However, I don't think that I'd try that w/the rabbit in the movie. If the Holy Handgranade couldn't "snuff it", I do not think a wrist rocket would either.
With plenty of practice over the years, I was able to place a round stream stone in a tennis shoe sized target at 30 yds. Lots of fun and effective for small game out to 25yds.
As to your questions:
If you jacketed a stone, consisting of hard minerals, that you took time to jacket w/copper after forming the rock to the appropriate dimentions, it may or may not leave the barrel. Even jacketed bullets contain lead. The propulsion (ie. velocity) and accuracy would be determined largely upon the seal that is made by the bullet. Bullets require lead because it is soft enough to conform to the groove/lands of the barrel. Hard minerals would not do this and may become wedge, causing the barrel to "back fire" or KB. Shotguns have used steel shot effectively. I believe that if you used an appropriate sized barrel of a muzzel loader, you might be able to shoot rocks, nails, clams, or any such critter of suitable size. Again, I believe that it would be very limited as to accuracy, range, and safety.
One of the most effective stone projectile weapons in history for medium range combat was demonstrated by King David of Israel as a young man; eventually to become a great grandfather to Messiah. He was expert w/the simple sling. This consisted of a short stick w/a long leather strip or rope connected on one end to the end of the stick. There was a pouch to hold the stone out at the full extension (half the length) of the cord. Having gained his confidence from a promise given by the Lord to Israel, he decided to exercise faith in that matter of Bible doctrine. Having chosen smooth brook stones, one for Goliath and each of his brothers, he went to the battlefield. Upon reaching the greatest Philistine warrior he said, "...I will smite you, and take your head from you; and I will give the carcases of the armies of the Philistines this day unto the fowls (vultures) of the air and to the wild beasts of the earth; that all the earth may know that there is a God in Israel!" He then proceeded to chamber a 1.0+" caliber stone into his days' version of the handgun/possibly shotgun. He used a simple, yet highly effective weapon, by the providential grace of God, to hit the only open spot in the heavily armoured steriodal freak of nature. The stone sunk into the frontal lobe of the freak's brain and David then proceeded to sever Goliath's neck, divorcing the head from the carcass with it very own sword! I Samuel 17:46-54
Sorry to go on so long, Oleg. I hope that you aren't sorry that you asked the questions now. This is one of my favorite passages from the Prophets and the subject happened to remind me of it.
robert