Because copper is lighter than lead the bullets have to be longer to make weight. Like Gamestalker said they will have to be seated deeper.
I stay away from them because of two reasons, the price and the extra length usually messes with the riflings in the barrel, degrading the accuracy of your gun, sometimes the solid coppers, and Homogeneous solids (which are also lighter than lead) will be a lighter weight so the length will still in range of what the riflings can stabilize.
I use to shoot the solid coppers but they seamed to copper foul my barrels faster than standard bullets and I never saw any big improvement in anything to justify the extra cost.