I'm not real sure about the copper jacket thickness on lighter bullets vs. heavier bullets. But what I do know is that bullet manufacturing companies have figured out that a faster bullet has a better chance of opening up, or mushrooming, i.e. rifle ammo. So they have applied this knowledge to handgun ammunition. While you can't get past the laws of physics, you have to be creative. If you want more speed, you can't add more powder, or a faster burning powder, so you just make a lighter bullet! I also cast bullets, and trust me, cast bullets will give you all the penetration you want, and then some! I shoot a .38 load designed for my Ruger SP 101. It's a 158 gr. DEWC that is extra hot, or .38+P+. I won't shoot it in any of my .38's! It's a proven fact that a bullet that doesn't expand, can only make a hole as large as it's original diameter. A .38 that does open up, or mushroom, is making a hole that is closer to .50. The bigger the hole, the faster the air escapes, or the blood! Hard cast bullets just pass right on thru, making them perfect for shooting small game, and yet still a good choice for large game, and even dangerous game, because they penetrate, go deep, and break bones.