If you're trying to thin the chunks down into a preform, the antler is the way to go. You don't need that unholy copper anyway, antler is better.
If you're trying to knock spalls off the chunk to make small points from, a hammerstone is what you want, a big moose antler is good for spalling, but deer antler usually lacks the weight to knock off big spalls. Find a rounded hammerstone a little smaller than fist-sized, preferably a gritty river-smoothed stone that's hard enough to take the impact, but soft and gritty enough to "grab" the edge. Find an edge of your flint that's strong, but less than a 90 degree angle, and strike a glancing blow with the hammerstone, aiming for just a bit in from the edge. Let your swing follow through. You don't want to strike straight in, the glancing blow will take off a good spall if you get the angle right.