Dogman,
Here is the simple answer to your delima. And the only way YOU will ever find the truth. And after all, isn't that what your looking for?
So, just buy 'em all. Yup. Buy all three barrels. Then get the dies and reload 'em all. Then you can go to the range and test different loads. You can fill up old tide jugs with jello and shoot 45-70 500 grainers at 100 yards and see what happens to 'em. You can do the same with the 30-30.
But, and this is fair warning, you will then read something about the balistics of a cartrige called a 7-30 waters. And then you will find out it's parent case is the 30-30. So....you will realize you have the brass....you will buy the dies, the barrel and 7mm bullets....then off to the range again.
Your grass will remain uncut for weeks at a time. Your garbage will pile up. Your neighbors will not like you.
So, what I'm sayin is that part of your life will stay pretty much the same as it has been.
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Just buy all three, you won't regret it.
Dave.