I've had the opposite experience with 243win brass. I bought Lapua brass and was not able to get consistent small groups like I do with Winchester brass. I thought it might get better on the second firing, but that was not the case. I went back to Winchester brass.
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if you tried using the same exact load with your Winchester brass as your Lapua brass it probably wont work. Been there done that pal. When I switched from Winchester to Lapua, I had to work a whole new load up for that brass. Once that was complete, the Lapua shot better and more consistent than the Winchester. And the Lapua cases are still working well.