Using a maxium charge in a rifle case will there be much of a pressure difference between a once fired neck sized case, once fired full length resized case, also new case full length resized or necksized.

No, not really.

Where a pressure difference comes with neck sized cases
is in say....

a 30-06 with a neck sized case that's presumably blown out to fit the chamber and in this case you can fit just a bit more of a slow burning powder (like Reloader 22) into the case.


There are similar situations basically where the "maximum" charge winds up being a compressed charge.

And in certain cases where it's just not possible to put "too much" of a specific propellant into the case (like RL22 in a 30-06)

In those specific cases LESS propellant isn't as good as as much as you can make fit.

It's kinda weird when you go UP half a grain and see the muzzle flash get SMALLER.

In other situations what people are looking for is to size the brass as little as possible
(sizing the neck is prtty much unavoidable)

And though some have reported no advantage in accuracy with neck sized cases that are in essence "fireformed" to fit a particular
chamber others report somewhat different results.

I have rifles where it doesn't matter and some where it does.
In general rifles with tight chambers are more forgiving of brass that's sized more (because obviously there's less "wiggle room")

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