“am afraid that i will be getting way too much excess spacing” You want to turn necks, turn necks. But as Jim K. said “neck turning is not always necessary”.
Other reasons for truing the neck, the neck could have more brass on one side than the other, having more brass on one side than the other bothers a lot of shooters. Then there is that part where it has not been decided if the bullet is pushed out of the case first, or if the neck expands first then the bullet makes the jump to the lands.
Lag, the bullet is setting still, it is not moving then suddenly and without warning pressure builds, back to the neck, the neck expands effortlessly, the bullet is setting still. Back to the neck releasing the bullet, could be the bullet is released first when the neck expands, meaning the neck expands first.
Excessive spacing? Between the neck of the chamber and case neck? A friend ask for help, he builds bench rest type rifles, he suggested the necks in his chambers are too large, I suggested he get off the Internet or avoid Internet reloaders. I ask him about the accuracy of his rifles and it came down to one hole groups, then I ask him what was his goal, “Half hole groups?” Not my barrels, not my cases, not my reamer, I boxed up a few tools and went for a visit.
The diameter of his necks for loaded rounds measured .335”, neck diameter of fired cases measured .344”, with no shortage of 30/06 cases we started forming 308W cases using 30/06 cases. When finished his neck diameter for unfired/loaded rounds was .341” (before firing). We reduced the difference in diameter between his chamber and case necks. Next, we are starting on forming cases using anything as long or longer than a 338 Winchester Magnum, problem I can not find my short 300 Win mag forming die.
Effort, time and money, check with RCBS to determine if the still have neck sizer plugs in different diameters, when sizing cases use different neck sizer plugs for different neck diameters. (OR) Learn to form cases, a 308 W formed from a 30/06 case will have a thicker neck, when sizing the formed cases start with the smallest (in diameter) expander plug and work up. The Forming/trim die is also a good tool for determining the length of the chamber from the bolt face end of the chamber at the end of the chamber neck.
I have ball micrometers, blind end micrometers and transfers, and if a reloader can determine the diameter of a bore with a soft lead ball the reloader can determine the diameter of the neck diameter in the chamber, with a soft lead ball.
Then there are expanders, unknown to the reloading world, meaning, there is no rule that says the lead has to be in the form if a lead ball, it can be in the form of a cylinder.
F. Guffey