While longer throats do allow more velocity, it's not as much as many people believe, mostly because they've never performed their experiments while actually testing pressures at the same time. This is true whether the longer throat is used to seat bullets further out, or simply to serve as "freebore," as in the Weatherby rounds.
The basic rule of thumb is the 4-to-1 Rule I developed by crunching the numbers from a bunch of pressure-tested loading data: Any increase in powder capacity results in about 1/4 as much increase in potential velocity--at the SAME pressure. This was confirmed in a different way by the late Don Miller, the guy who developed the rifling twist formula used almost universally these days.
Let's say somebody lengthens the throat on a .30-06 by 1/4 inch. This results in .0186 of a cubic inch of extra powder space--.25 inch times the cross-sectional area of a .30 caliber bullet, .0745 inch. This amounts to about 5 grains of extra powder space, which sounds like a lot until you consider the .30-06 case holds about 60 grains of powder with a 180-grain bullet seated. The longer throat thus results in about an 8% increase in powder room--and 1/4 of 8% is 2%.
If the .30-06 is capable of 2800 fps with a 180 (and it is, even at SAAMI pressures, with some powders), then the extra 2% will up the potential velocity to about 2850--again, at the SAME pressure.
But if we increase the pressure from the SAAMI maximum average pressure (MAP) of 60,000 PSI to 70,000 PSI (about where many "pressure signs" used by handloaders typically start appearing) the velocity can rise to around 3000 fps--or even a little more, depending on the powder.
Is this safe? Absolutely--in those conditions, in that rifle. But pressures are never constant, even with today's far less temperature-sensitive powders, the reason SAAMI (a collection of ammo and firearms manufacturers) has yet to approve a MAP of over 65,000 PSI: There are too many variables to expect 70,000 PSI to work safely in every rifle, under all conditions.
Which is exactly why I conducted the experiments with the .223 Remington, loading it up until "pressure signs" appeared, then backing down to the last load that didn't results in any such signs. Obviously that load was safe in the pair of .223s I tested it in, but the velocities were so close to those quote by fans if the .223 Ackley Improved there was no practical difference. Which indicates the velocities achieved by .223 AI fans are almost entirely due to more pressure, not the little bit of extra powder room.
And a little bit extra powder room, whether in freebore or seating bullets out farther, is what throat lengthening is all about.
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