Developing Load Data for Wildcats:
Starting from Ground Zero
Once you have a data set to work with, loading a wildcat is no different than loading any other cartridge. You size cases, prime, drop the charge, and seat a bullet. Pretty cut and dried. But what do you do if you don't have any data? You have to determine what type of powder to use, how much to use, and with what bullet weights. You have to decide on whether to use standard or magnum primers, and how long the barrel needs to be to get the performance you are aiming for. If you are new to the reloading game, or have never worked with a wildcat, it can be a very intimidating task.
The following is one method that can be used to determine a baseline starting point for just about any wildcat. It requires using basic math skills including the use of some relatively simple formulas you can get from the web. You also need to take good notes, and take you time so you don't make silly errors. You can use software programs like QuickDesign and QuickLoad, but the learning curve for those pieces of software is just a intimidating if not moreso, since you need computer skills to use them right. You may thus find my method easier, even if a bit more time consuming.
First, of course, you need to know the basic dimensions of the case of your wildcat. Overall length, head to datum line length, case capacity in CC of water (measured to case mouth), inside neck diameter, etc. If you have a chamber drawing for your cartridge that should have everything on it you need except for the case capacity. You'll need similar data for the range of bullets you plan on using, as well as basic information on the barrel itself like land and groove diameters, length, etc.
To make this work, you need to calculate some things once you get the data mentioned above gathered up. You will determine case capacity (both to the case mouth and with a nominal bullet weight seated). You will determine expansion ration based on the case capacity and the barrel length for the caliber in question, and the same for a reference caliber. You will also use the Sectional Density and Ballistic Coefficient of bullets in the wildcat caliber as well as the reference caliber.
To determine case capacity, take a case (newly formed if you have it but fired is OK) and put a spent primer in the primer pocket to seal it from leaking. If the case has been fired, partially size the neck enough to hold a bullet. Take a case with the fired primer in place and weight it empty, record the weight. Fill the case with room temperature water to the case mouth and level it off (no miniscus). Weigh it again and record that weight. Subtracting the first empty weight from the filled weight will give you the gross case capacity. Record that number.
Weigh a bullet, and scribe a deep scratch on the length of it from base to ogive. Now, take the full case and slowly seat that bullet to a nominal depth such as the cannelure if it has one, or roughly to that point if it doesn't. Seating pressure will force the excess water out between the bullet and case via the scratch you made. When it is seated to the right point, weigh the case and record. Pull the bullet and remove the spent primer. Subtract the bullet weight from the weight you just recorded. The result is the capacity of the case in terms of powder space.
Next step is to determine barrel volume. To do this, close the bolt on an empty chamber. Drop a brass rod or wooden dowel down the barrel until it bottoms out on the bolt face. Mark the length at the muzzle. Use you your case diagram to determine the case length from bolt face to case mouth and subtract that from the length you measured for the barrel. That gives you the actual barrel length from case mouth to muzzle. Use the formula for the volume of a cylinder to figure the volume of that length of barrel. I usually add the bore and groove diameters together and divide by 2 to get the average to use for the diameter in the formula. Record this figure. To determine expansion ration, you take the case capacity in cc's and divide it by the barrel capacity. You will need this number, so record it.
To summarize, we have the following: case measurements, case capacity (powder and to case mouth), barrel volume, and expansion ration. The next step is determining our reference caliber. Here's where you need the sectional density and ballistic coefficient.
To do this part right you should have several reloading manuals from different manufacturers. Most manuals have a section in the front that gives basic data on each bullet from that company. Things like weight in grains, length, etc. Start looking sectional density numbers for each bullet. Make a note of the ones that are the same or closely similar to the bullet you plan to use. Now compare the coefficient for those you selected, eliminating the ones that aren't close. What you end up with will have roughly the same coefficient and density, regardless of caliber, so they should all look very much alike (flatbase spitzer, roundnose, boat tail, and so on). Also, pick cartridges that are as close as possible in size.
Now, look at each of your reference candidate cases and the max charges in the tables for each one. You're going to figure the expansion ration based on the max charge and the barrel length for each caliber just like you did for the wildcat. Try to get a case for each caliber to get an accurate capacity. If the expansion ratios you get are close to your wildcat, then that cartridge and bullet is a candidate for a reference caliber. What you'll find when you compare the ballistic tables for each to one another is that they all have roughly the same ballistics, due to similar density, coefficient and expansion ratio.
Now, you can go to the loading table for your reference cartridge and look at which powders are being used. Use the slower powders that are shown, and those same powders will likely work in your wildcat. Of course, you have to realize a few things at this point. If you have a barrel of X length and Y caliber, the powder you use will have a certain burning rate based on case capacity, bullet weight, etc. Use too fast a powder, and you'll hit the pressure ceiling before you get to the optimum load density. Too slow a powder, and you'll run out of case capacity before reaching that optimum point. Homer Powley determined decades ago with IMR powders that 80% load density was about the best point for starting loads. So, if you cartridge has a powder capacity of say, 60 gr., 80% of that would be 48 gr. Compare that to the load tables. You'll get a rough estimate of the 80% point with the loads shown. The powders that meet that criteria in the reference caliber will be suitable for your wildcat. At this point, you can load up a test ladder and do load workup just like you would with any known cartridge.
I have used this method on several wildcats over the years, and it works. You have to be patient, and make sure you do the math correctly, but it isn't particularly difficult. I used this for a .375/.338 a number of years ago. The goal was to reproduce the performance of the .375 H&H in a standard length action (I used a Ruger M77R in .338 Mag as the base). I not only matched the ballistics of the H&H, but due to the more efficient case design I exceeded them at every level. The 300 gr. loads beat the best factory loads in that weight by almost two hundred feet per second at the same pressure levels.
This method will work for any wildcat. The key is the research and finding as many suitable reference cartridges as you can. It gets you where you need to be for starting loads, but it also makes you intimately familiar with your wildcat and how the various relationships all play a role in getting to your goal.