Nope, as mentioned before, the Silver Coin works perfectly, it's the copper coin that's the problem.
GameRegistry.addShapelessRecipe(new ItemStack(ModItems.copper_coin, 5), "ModItems.silver_coin");
// An Item and not an ItemStack
GameRegistry.addShapelessRecipe(new ItemStack(ModItems.silver_coin , 5), "ModItems.gold_coin");
// Also an Item and not an ItemStack