Well, I see that while I was gone, Dutch gave you the right slant -- as far as he went. Let's go further.
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<br>First, let's get the words right. "Overbore" isn't the word. The basic term is BORE CAPACITY.
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<br>The basic term is TWO words, both critical to meaning and understanding. A cartridge is UNDER bore capacity, right AT bore capacity, or OVER bore capacity. "Overbore" is a meaningless nonword in this application.
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<br>A cartridge is at or under bore capacity as long as there's a canister powder reasonably available that's SLOW enough to fill the case WITHOUT raising peak pressures above the safe maximum for the firearm that fires that cartridge.
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<br>Most cartridges are well under bore capacity since the introduction of powders slower than IMR 4350. Many that used to be over bore capacity are now UNDER bore capacity, since the newer, slower powders (4831 through 50BMG) make it not only possible but easy to fill the case (hence the word "capacity") without the resistance (hence the word "bore") to the propelling expansion of that powder's gas being great enough to raise the peak pressure of the expanding gas to a dangerous level.
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<br>IOW, many cartridges that were OVER bore capacity in Ackley's day (because the slowest canister powders available then were 4350 and the just-introduced surplus 4831) are no longer over bore capacity if a caseful (100% of the net capacity under the bullet) of, say, IMR-7828 doesn't produce dangerously high peak pressures in that cartridge, with that bullet.
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<br>Today, a cartridge is OVER bore capacity if a caseful of the slowest available canister powder is excessive (produces dangerously high peak pressures) for the firearm that fires that cartridge. I don't know of any. I haven't explored the slowest, which is probably Hodgdon's 50MG. My guess is that a .30-06 necked to .17 would be over bore capacity even for .50 machine-gun powder, and I'm sure that a .17-.50 MG would be 'WAY over bore capacity.
