Hi there and welcome.
Are you talking about the actual round or the rifle? I think you mean the round in which case...
...not much. The 6mm Remington has a slight ballistic advantage over the much more popular .243 Winchester due to a slightly larger case capacity. However, there is no discernible difference in real world hunting applications. The longer case neck of the 6mm Remington is considered desirable by handloaders.
I will be here if you have a follow up comment or question.
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