NO! NO! "Copper BB's are copper-plated steel balls-they could RUIN your rifle barrel! Go to a gun shop and buy a bag of LEAD SHOT! I'd use #4's!

You don't say what caliber your rifle is, but if you made up some paper cups of just the diameter to slide freely down the bore, put a cap in one end, filled them with shot, then a cap on the open end, you could load the shot into the gun in these paper "cartridges", after charging the gun with powder. If you make the paper cup several sheets thick, it might help keep the whole shot charge from being spun by the rifling. If the shot charge spins, you will definitely have a "scattergun" all right! Maybe one with a circular pattern in which all the shot strikes around the edges and none in the center-but you will only know this by shooting. If your rifle is a .45, you could probably use .410-ga. plastic shot-cup wads made for loading .410 shotgun shells in it. A .54 or .58 might work with 28-ga plastic shot-cup wads.... Someone might also be making plastic shotcup wads for other ML rifle calibers too, but I don't know of any right now.

In lieu of a plastic shotcup, perhaps just a long sabot could be used as a shot cup, with a "Wonder Wad" seated on top of the sabot as an "over-shot" wad, to keep the shot inside. The thing is, you want to keep the shot from touching your bore for two reasions A: To prevent the massive leading Caygad is talking about; B: To reduce the tendency for the rifling to set the shot charge in a spinning motion, which will cause it to disperse like a lead tornado when it leaves the muzzle............. I would load at least 1/2 to 3/4 Oz. (218 to 330 grains by weight of pellets so you have some kind of a pattern out tho 20 yards or so)

(A .50 is around "37 gauge"-37 ea., 185-grain balls to the pound. A 28 gauge is a .55 caliber, or thereabouts.)