I am strictly interested in straight-walled pistol and revolver cartridges. With so many different bullet shapes, styles and nose lengths, it is possible to have two different bullets that weigh the same, have the same bearing surface, and yet can be seated to vastly different depths. This greatly effects case volume behind the bullet, and because of that, pressure.

Is there a formula which will predict or approximate the reduction in pressure when a bullet is seated out for a longer OAL? Here is an example from my own loading. The Hodgdon Data Center shows a 45 Colt load of the 225 gr Hornady FTX bullet seated to 1.600". I have a 225 gr cast bullet with the cannelure placed so that the overall length when crimped is 1.655". This long seating results in a case volume behind the bullet increase of 10 full grains, from 23.7 gr to 33.7 gr of water volume. This reduces the case fill from 49% to 34.5% with the same load of powder.

How much does this reduce pressure? And how much more powder should be added to achieve the same pressure? I will still start 10% below max and work up to max loads, but how much more should I start with?

I have Quickload and there is simply too many variations in reloading components to give a accurate answer.

The deeper a bullet is seated it lowers the case capacity and raises chamber pressure.

And when you seat a bullet longer it will raise the start pressure.

That being said individual bullets can be seated longer and only restricted by the revolvers cylinder or pistols chamber.

In the Speer #14 reloading manual in the rear is a chapter on the pressure and velocity differences in .357 revolvers.

Bottom line this is why the reloading manuals tell you to start low and work up your loads.

Read the link below and the postings by Wobbly, and this applies to all pistols.

How to determine Max OAL for a CZ Pistol
http://www.czfirearms.us/index.php?topic=34225.0

The answer is yes, what you describe is true.
But I never worried about it and never had any issues with any of the revolver cartridges I have loaded. Go by what the loading manuals say to do.

Bullet seating depth generally has a fairly small impact on pressure and velocity. That's because the "combustion chamber" is NOT the volume of the cartridge case behind the bullet, but rather the entire gun chamber up to where the bullet engages the barrel lands (and beyond, for that matter, but in this instance we're talking about the pre-barrel volume).

Changing the bullet seating depth doesn't change the over-all volume of the combustion chamber. However, other factors like additional case neck grip on the bullet and compaction of the powder charge can and does slightly effect pressure and velocity. This is particularly true with handgun cartridges as can be illustrated in the difference in velocities between uncrimped case mouths and heavily crimped case mouths where the grip of the crimp -- in addition to the case neck friction and the bullet's inertia -- must be overcome for the bullet to start moving and thus expand the combustion chamber.

All of those factors -- deeper bullet seating, additional neck friction, and mouth crimping -- tend to increase pressures. However, the depth of seating probably has the smallest impact.