There is no need to try to pack more powder in the case than it will hold with a little compression. Shooting a carbine with a full load and 500gr bullet will not be fun. Have you slugged the bore to find groove size? If your bullet is less than groove size, it will not shoot accuarately, no matter what weight it is. If its a carbine converted from percussion, then a 450 gr bullet should shoot well if sized and lubed properly. What kind of rifle is it - military, sporting? converted 1863 model or 1868 or 1874 model? have you measured the twise rate as well as groove size? These are all factors that go into determining a proper bullet and load.