My experience is the same as your experience, the copper-plated bullets do not shoot as well as cast and lubricated bullets. This is even with moderate target loads. My thought is the copper plate is not even and the stress of the firing causes the copper plate to crumble in the barrel. As the bullet exits the barrel, the copper plate sloughs off and unbalances the bullet. I was never able to recover these bullets as the bullets crumbled when impacting the backstop.
When I expressed this opinion on anther forum, I was almost instantly flamed by someone. The quickness of this action caused me to think the manufacturers keep someone on the payroll to monitor the web and defend their product.
For target shooting with a .38 Special, I shoot the RCBS 148 grain wadcutter with 2.7 grains of Bullseye and this works well. The load outperforms my ability to hold. It will shoot well to 50 yards, but not to 100 yards.
For .45 ACP and .45 Colt, I shoot the SAECO #954, 235 grain round-nose flat-point. I also shoot the H&G 200 grain semi-wadcutter with the above calibers.
Country boy from Illinois, living in the Magical Pacific Northwest