CAUTION: The following post includes loading data beyond or not covered by currently published maximums for this cartridge. USE AT YOUR OWN RISK. Neither the writer, The Firing Line, nor the staff of TFL assume any liability for any damage or injury resulting from use of this information.
Long ago, for high power rifle, the NRA's technical guru, Wm. C. Davis, Jr. advised reducing charge weight by a grain of powder for each additional 16 grains in case weight in order to keep velocity constant in the .30-06 with IMR rifle powders. QuickLOAD confirms this seems to come close with them and with a good many other rifle powders in a good number of high power cartridges. Since cartridge brass has a density of 8.53 grams/cc, it meant about 0.53 grains of powder for each grain difference in case water capacity under the bullet (as distinct from case water overflow capacity, where the water fills an empty case level at its mouth). That, however, does not keep the pressure constant. Because adding more powder means you make more total gas, the muzzle pressure is raised, resulting in a larger portion of the bullet's acceleration occurring later in the barrel and less of it near the peak. So, for velocity to be the same, the peak pressure has to come down, and because the bullet has less velocity accrued at the pressure peak, it begins its trip down the rest of the barrel with less velocity and so, even though it corrects the velocity deficit further down the bore from the increased late-barrel pressure due to the greater volume of gas, the key word is "later", so barrel time is increased. To keep peak pressure constant takes a little more like 0.65 grains of powder per grain of volume increase. But that raises velocity and makes the barrel time shorter. If you want constant barrel time, the increase will be somewhere between those two numbers. Whether constant velocity or constant barrel time is more important to accuracy depends on the load tuning and range you are shooting at. Decisions, decisions. Got a headache yet?
Another way to look at Davis's rule is by volume. Since the average IMR powder has a bulk density a little under 0.9 grams/cc, he was changing the percent powder volume by a factor of 0.6% for each 1% change in case water capacity under the bullet.
So, now we get to the handgun. Does Davis' same rule apply? No. It quickly becomes dangerous. It really assumes a nearly full (85% or so) case to start and that the change in grain weight is a small percentage of the total charge. If the cartridge has a lot of space and a small charge to start, that rule quickly overloads it. If we had 6.9 grains of Unique behind the FTX as a starting point at 1.600 COL and 43 grains total case water overflow capacity, QuickLOAD predicts a pressure of 12,646 psi. A 10 grain increase in water capacity accomplished by seating the FTX to 1.847" (seating depth 0.2387"), by Davis's rule would be a 5.3 grain increase in the Unique charge weight to 12.2 grains. QuickLOAD says that takes us up to 23,826 psi, exceeding the SAAMI maximum proof pressure of 22,000 psi. The grain guess simply does not work with small charges.
But then, let's try Davis's rule converted to a volume change factor of 0.6. Going from 23.7 to 33.7 grains is an increase of 142.2%. Taking just the increased portion, 42.2% and multiplying by 0.6 gives us 25.3%. If we increase the 6.9 grain Unique powder charge by 25.3% (a total of 125.3% of 6.9 grains) we get 8.65 grains. Substituting that into QuickLOAD (which allows hundredths of a grain) the pressure prediction for the longer seated bullet is 12,748 psi. That's really close enough to be the same.
I didn't find Unique in Hornady's loads for this bullet, but doing the same thing with Longshot, which is there, also got a very close match. What is happening here is juggling the increase in space with the total gas volume and the rate of expansion. That is, the rate at which the volume behind the bullet doubles gets slower as the case capacity goes up (smaller expansion ratio). These factors work in opposite directions, but since the rate of acceleration of the bullet due to pressure is unchanged, you can balance them. With the peak pressures the same, powder burn rate and curve shape won't change appreciably, greatly simplifying the underlying math model.
The fly in the ointment.
Note that all the above was with the same bullet. The lower start pressure of the cast bullet will lower the peak a little bit, but if you try this, I would keep that as a safety factor.
