Powder Selection and Case Capacity

For those who own a rifle chambered for an obsolete or a wildcat cartridge, selection of the appropriate powder for reloading can be a problem. For high pressure cartridges, newer software such as QuickLOAD and even the older Powley Computer can make suggestions for powder and charge weight which are usually close enough you won't get hurt. For older rifles and chamberings, more care must be taken. Older reloading data is often for powders no longer made, and much of that data was never properly pressure tested.

This file describes a way to use modern reloading data to help select a suitable powder. With the aid of a chronograph, one should be able to work up a reasonable load.

Concepts

Bullet "sectional density," or SD, is a well known concept among shooters. It is commonly expressed as the bullet weight in pounds divided by the square of the bullet diameter in inches. From the standpoint of the high pressure gases from the burning powder, this number represents how slowly the bullet will accelerate, or get out of the way. The greater the SD, the slower the acceleration.

One can also divide the net case capacity (that below the seated bullet) by the bore's cross sectional area and get a number similar to bullet SD. It represents the "inches of bore" available for the powder, analogous to the space behind the bullet in a muzzleloader. Loosely, this length represents how fast pressure will be relieved by a given bullet acceleration. I'll call this length the Relative Capacity (RC). RC represents the net case capacity replaced with a bore section cylinder of the same volume:

Also marked in the drawing is the distance the bullet travels, labeled T. The greater T is, the greater the muzzle velocity will be, since the gases have a longer distance over which to push and accelerate the bullet. The bullet travel is often stated in terms of the "expansion ratio," or ER. The ER is defined as the volume behind the bullet as it exits divided by the volume behind it at ignition. For a given peak pressure, the greater ER is the greater the velocity will be. In terms of RC, the ER is simply:

ER = ( RC + T ) / RC

Lastly, there is the concept of loading density, or LD. This is commonly stated as the charge weight divided by the net case capacity in gn of water. The LD is basically "how full" the case is of powder.

So how do these concepts come together to select a powder? Simplified, if RC, SD, and LD are the same, the pressures produced will be the same. If you can find a published load in another cartridge with a similar RC and SD, that powder at the same LD will give similar performance in your cartridge. If the ER is the same, you'll also get roughly the same fps.

I was able to verify the muzzleloader analogy and associated ideas as described in the next section. After first writing this article in 2003, I began watching for earlier expressions of these ideas in internal ballistics papers, and in 2009 I finally found it. In 1903 in the Kynoch Journal, R.H. Housman described his long running experiments along this line of thought. They are compactly presented in the Textbook of Small Arms, originally published in 1929 by the War Office of the U.K. Also in 2009, I was looking over again P.O. Ackley's Handbook for Shooters and Reloaders and found he had already given a name to what I here call the Relative Capacity. Ackley called this ratio the Bore Capacity, but he didn't develop the idea as far as Housman did. Since the abbreviation BC would be confusing, I'll stick here with RC for now, although HC would be appropriate.

Confirmation

All of the interior ballistics software I've tried exhibited this behavior. These include the Powley Computer, QuickLOAD, Load from a Disk, and LoadTech. (The sole exception is likely to be Steve Faber's NABM. Sadly, this sophisticated simulator is no longer supported.)

One can also find this relationship by scaling the data in load books, such as Lyman's 48th or Hodgdon's Annual, and that is thrust of this article, below.

The equations used in commercial software are proprietary, but those for the Powley Computer have been published. The Powley Computer tries to select an IMR powder which when loaded for a nearly full case produces a chamber pressure around 44,000 CUP. The formula for the powder selection index, or "quickness," can be written as:

20 + 77.5 / sqrt( RC * SD )

Here, a result of 100 represents the burning rate of 4350, as measured in a laboratory test, and higher numbers represent faster powders, with 4227 being a 180. It is worth studying William Davis's comments regarding the predicted quickness values for various IMR powders.

Note that RC and SD receive equal weighting in the formula. Shorter RC's and lighter SD bullets both increase the needed burning rate. This is a simple formula, yet it does a respectable job of selecting powders over a wide range of case sizes and bullet weights.

As to why Powley's formula works, note it has RC and SD inversely proportional. Say you want to double the RC. The formula predicts that to use the same powder, you'll need a bullet of half the weight. Doubling the RC will, with a given pressure curve, double the rate of gas generation, for there is twice the powder behind the bullet. To keep the same pressure curve, the bullet must accelerate twice as fast in order to create volume twice as fast, thereby accommodating the doubled gas production. Thus the bullet's inertia, as represented by the SD, must be half.

A map of the powder selection in the Powley Computer is:

Problems

Powley's Computer gives fair estimations of cartridge performance, and one can use it to demonstrate the effects of small changes in SD and RC. Similar results are found with QuickLOAD. For cartridges proportioned like the popular .30-06 using powders similar to 4350:

               Powley       QuickLOAD
RC     SD        fps           fps

3.3   .280    2566 (-46)    2574 (-44)
3.3   .270    2612          2618
3.4   .270    2633 (+21)    2631 (+13)

Note that fps is sensitive to RC and is very sensitive to SD. Across calibers, it is difficult to match SD exactly, and across cartridges, it is difficult to match RC exactly. Because of this sensitivity, there is little hope one can use load data from one cartridge to predict the fps of another cartridge similar to it.

The inability of this method to predict fps is amplified by those fundamentals which make the internal ballistics of small arms nearly impossible to predict. Small changes in bullet construction can greatly change how readily the bullet enters the bore. The slower the bullet enters the bore, the greater the pressure rises behind the bullet. The greater the pressure gets to be, the faster the powder begins to burn. This can all get out of hand rapidly. Another big factor is the primer, for it determines the initial rate of pressure rise, and thus how fast the burning accelerates. Powder lots can vary remarkably in speed, compounding a complicated problem.

Do not attempt to use RC for anything other than powder selection. It will not predict fps and charge weights reliably.

Fortunately, as the Examples below might show, powder selection is not nearly so sensitive to RC and SD. The width of the bands in the powder selection map above hints at this. The relative narrowness of the bands with the faster powders hints why software such as the Powley Computer can sometimes suggest a powder too fast. I would not use RC for powder selection if the speed needed is faster than the 4198 class.

Examples

The relationship of peak pressure to RC and SD can be seen by scaling the data in load books. As mentioned, fps predictions are hardly spot-on, but powder selection works quite well.

Consider again the above map of powder selection from the Powley Computer. While the .17 Rem and the .243 Win are not usually considered alike, they have nearly the same RC. At an SD of .145, the common bullet weights are 30 and 60 gn, respectively. The map shows the powder indicated is in the 4895 class. Limiting oneself to stick powders (per the original Powley Computers), Hodgdon's 2004 Annual shows the best performance for both with Varget (and H4895 not far behind), a powder indeed within the 4895 class. These numbers are for a pressure of about 51,000 CUP and a 24" barrel:

             gn   RC     SD       Varget      LD

 .17 Rem     30   4.3   .145    22.5  3742   .92
.243 Win     60   4.4   .145    42.7  3816   .85

Unfortunately for my argument, the LD's are different, confirming that RC cannot be used to predict LD, for if nothing else, lot variations will prevent it. Note that while the fps aren't too far apart, they are enough different to demonstrate that RC can't predict fps either. Again, the same powder was the best performer in both these cartridges of similiar RC and SD.

Moving from varmints to big game, one finds the .243 is similar to the .300 H&H. Powley's predicts the 4350 class to be a good choice, and Hodgdon's data shows this to be so for both:

             gn   RC     SD       H4350       LD

.243 Win    107   4.2   .259    39.8  2853   .83
.300 H&H    180   4.2   .271    69.0  2990   .90

Here, despite the greater SD, the .300 was the faster, but it ran at a higher pressure than the .243, by 3400 CUP. Still, the 4350 class was the best choice for both, despite some difference in SD.

At the beginning of this article, it was mentioned one might use this method to help select powders for older cartridges. The .33 WCF hasn't been loaded by the factories for over 50 years. Consulting the table below, one will find it not far from the .25-35, a cartridge with similar pressure ratings, but for which there is a decent amount of modern load data. The 200 gn FP in the .33 has an SD of .250, which corresponds to a 116 gn bullet in the .25, very close to the standard 117 gn RN loaded in it. Checking Hodgdon's data, one finds Varget is the best stick powder. Other cartridges of similar pressure and proportions include the .300 Sav, which Lyman 48 shows performing best with Varget. The older reloading manuals (eg. Hornady) report IMR's 4895 and 4064 giving good performance in the .33 WCF, and these powders are indeed similar to Varget in burning speed, actually a trifle faster. Hodgdon got over 2200 fps with Varget in the .25, similar to the old factory rating of the .33 WCF.

Lastly, some conversions are simple. The old .35 WCF scales nicely to the modern .35 Whelen, and the starting loads in Lyman's 48th are at the pressures the older cartridge ran at.

Caution

If you can find a cartridge with extensive, pressure tested data that scales well to your odd cartridge, you can use this method to help select a powder. You should use the primer tested in the load book, for it can have a big effect on pressure (although the lab running the tests might only notice primer effects if they are using piezo sensors for pressure). When working up your load on the chronograph, knock about 5% off your expected fps, to try to guard against the effects on pressure of bullet construction, etc.

Because the variability of bullet engraving, primer ignition, and powder lots are difficult to quantify, all commercial internal ballistics software simply ignores these effects. (QuickLOAD tries to deal with some of these, but not rigorously.) All such software shows the relationship between RC, SD, and LD discussed here, but as the examples show, pressure tested load data can show significant variation in fps. This implies these software are of limited accuracy. As with using RC and SD, such software can help you select a powder, but it cannot tell you how much of it to use, nor can it tell you what fps you can reliably expect to get at a given pressure. Always end load development at a chronographed fps below what the software predicts.

Cartridge Comparisons

Below is a table which can help you find cartridges which scale. The RC is a function of bullet seating depth, and this depth varies greatly. Each pair of listings instead shows two related numbers, both of which are in inches.

The first number shows the capacity of the empty case divided by the bore cross section. Subtract bullet seating depth from this number to get RC.

The second number can be more useful for finding a cartridge similar to yours. Added on to the scaled empty case length, is the length the bullet can protrude. How far the bullet sticks out is only important for repeaters, but the more the bullet can stick out, the more room there is for powder. Since bullets of like SD are about the same length, this total, scaled cartridge length is useful if like SD bullets are being used. Subtract the bullet's length from this number to get RC.

The listing is sorted twice, first in ascending scaled cartridge length and the second in ascending scaled empty case length.

Fortunately, there are numerous load books today which publish pressure and fps for both starting and max charges, giving one a large range of LD and pressure data. Using typical values for case capacity, one can estimate the RC and LD for this data. It can be tedious to compute these, but the Powley Computer on this site computes RC and SD, given the other cartridge dimensions.

If you wish to compute RC by hand, first measure (in gn of water) the case volume of a fired case before it is resized and then divide by 252 to get the cubic inches of water. Next subtract off the volume occupied by the seated bullet, using the bullet's diameter and seating depth. Lastly divide by the bore's cross section, which can be estimated as .773 times the square of the bullet diameter (this .773 is about 1.5% below the cross section of the bullet, to allow for the barrel's lands).

Hodgdon's Annual reloading manuals give extensive loading data, including pressures and velocities for both max and starting loads. Covering a very wide assortment of cartridges, such data is ideal for testing this theory. It is supplemented by more recent data on their web site. Similar data is found on IMR's site. Other sources for similar data are Lyman's 48th and A-Square's reloader's manuals.

                  Case and Cartridge Comparisons

                 by 2nd column        by 1st column

                empty    plus        empty    plus
                 case    bullet       case    bullet
                -----    -----       -----    -----
.357 Mag         1.08    1.38         1.08    1.38    .357 Mag         
.44 Mag          1.08    1.41         1.08    1.41    .44 Mag          
.30 Carb         1.13    1.52         1.13    1.52    .30 Carb         
.25-20 WCF       1.47    1.73         1.37    1.76    .357 Max         
.357-44 B&D      1.41    1.74         1.41    1.74    .357-44 B&D      
.22 Hornet       1.43    1.75         1.43    1.75    .22 Hornet       
.357 Max         1.37    1.76         1.47    1.73    .25-20 WCF       
.218 Bee         1.84    2.18         1.78    2.32    .375 Win         
.444 Marlin      1.92    2.27         1.84    2.18    .218 Bee         
.38-55 Win       1.86    2.29         1.86    2.29    .38-55 Win       
.375 Win         1.78    2.32         1.92    2.27    .444 Marlin      
.45-70           1.94    2.39         1.94    2.39    .45-70           
.25-25 Stev.     2.25    2.51         2.04    2.64    .35 Rem          
.221 Rem         2.14    2.57         2.14    2.57    .221 Rem         
.35 Rem          2.04    2.64         2.25    2.51    .25-25 Stev.     
.28-30 Stev      2.35    2.66         2.26    2.76    .38-56           
.17 Hornet       2.42    2.74         2.28    2.83    .356 Win         
.38-56           2.26    2.76         2.32    3.09    .358 Win         
.356 Win         2.28    2.83         2.32    3.16    .458 Win Mag     
.38-70 WCF       2.51    2.91         2.35    2.66    .28-30 Stev      
.30 WCF          2.43    2.95         2.36    2.98    .405 WCF         
.405 WCF         2.36    2.98         2.42    2.74    .17 Hornet       
 9.3x72R         2.56    3.00         2.43    2.95    .30 WCF          
.358 Win         2.32    3.09         2.44    3.14    .375 2.5" NE     
.303 Sav         2.59    3.10         2.45    3.38     9.3x57          
.375 2.5" NE     2.44    3.14         2.48    3.26     7 BR Rem        
.458 Win Mag     2.32    3.16         2.48    3.41     9.3x54R Finn    
.38-72           2.65    3.23         2.50    3.36     9x57(R)         
 7 BR Rem        2.48    3.26         2.51    2.91    .38-70 WCF       
 9x57(R)         2.50    3.36         2.56    3.00     9.3x72R         
 7-30 Waters     2.86    3.37         2.59    3.10    .303 Sav         
 9.3x57          2.45    3.38         2.64    3.44    .458 Lott        
.25-35           2.87    3.38         2.65    3.23    .38-72           
 9.3x54R Finn    2.48    3.41         2.76    3.51    .35 WCF          
.458 Lott        2.64    3.44         2.81    3.54    .300 Sav         
.307 Win         2.91    3.46         2.82    3.52    .33 WCF          
.35 WCF          2.76    3.51         2.84    3.69    .35 Whelen       
.33 WCF          2.82    3.52         2.86    3.37     7-30 Waters     
.300 Sav         2.81    3.54         2.87    3.38    .25-35           
.350 Rem Mag     2.92    3.55         2.91    3.46    .307 Win         
.17 Mach 4       3.29    3.61         2.92    3.55    .350 Rem Mag     
 8x72R           2.97    3.62         2.94    3.79     9.3x62          
.223 Rem         3.16    3.67         2.97    3.62     8x72R           
.35 Whelen       2.84    3.69         2.98    3.83     400/360 Purdey  
.348 Win         3.17    3.71         3.00    3.85    .303 Brit        
.219 Zipper      3.47    3.79         3.02    3.82    .308 Win         
 9.3x62          2.94    3.79         3.04    4.05     8x57J(R)        
.308 Win         3.02    3.82         3.13    3.90    .30-40 Krag      
 400/360 Purdey  2.98    3.83         3.13    3.98     400/350 NE      
.303 Brit        3.00    3.85         3.13    3.91     9.3x74R         
 6 BR Rem        3.21    3.85         3.14    3.99    .338-06          
.404 Jeffery     3.23    3.89         3.15    4.01    .450 NE 3.25"    
 5.6x50R         3.47    3.90         3.16    3.67    .223 Rem         
.30-40 Krag      3.13    3.90         3.17    3.92    .416 Rem Mag     
.22 Sav          3.45    3.91         3.17    3.71    .348 Win         
 9.3x74R         3.13    3.91         3.21    3.85     6 BR Rem        
.416 Rem Mag     3.17    3.92         3.21    4.07     375/303         
 400/350 NE      3.13    3.98         3.23    3.89    .404 Jeffery     
.338-06          3.14    3.99         3.24    4.24    .318 W-R         
.465 NE          3.34    3.99         3.29    3.61    .17 Mach 4       
.450 NE 3.25"    3.15    4.01         3.30    4.03    .470 NE          
.470 NE          3.30    4.03         3.34    3.99    .465 NE          
 8x57J(R)        3.04    4.05         3.36    4.21     9.3x64 Bren     
 375/303         3.21    4.07         3.41    4.33     7.62x54R        
 7x72R           3.43    4.08         3.43    4.08     7x72R           
.250 Sav         3.57    4.18         3.44    4.28    .460 Wea Mag     
.375 H&H         3.46    4.21         3.45    3.91    .22 Sav          
 9.3x64 Bren     3.36    4.21         3.46    4.21    .375 H&H         
.318 W-R         3.24    4.24         3.47    3.90     5.6x50R         
.460 Wea Mag     3.44    4.28         3.47    3.79    .219 Zipper      
 7.62x54R        3.41    4.33         3.53    4.39    .375 Fl. Mag     
 450/400 3"      3.59    4.34         3.55    4.53     6.5 Jap         
.505 Gibbs       3.65    4.35         3.55    4.37    .358 Norma Mag   
.358 Norma Mag   3.55    4.37         3.57    4.18    .250 Sav         
 7-08 Rem        3.62    4.39         3.59    4.34     450/400 3"      
.375 Fl. Mag     3.53    4.39         3.62    4.39     7-08 Rem        
 6.5 Jap         3.55    4.53         3.65    4.35    .505 Gibbs       
.30-06           3.72    4.57         3.68    4.58    .369 NE          
 7x57(R)         3.74    4.57         3.72    4.57    .30-06           
.369 NE          3.68    4.58         3.74    4.57     7x57(R)         
.338 Win Mag     3.85    4.69         3.85    4.70    .416 Rigby       
.416 Rigby       3.85    4.70         3.85    4.69    .338 Win Mag     
.225 Win         4.18    4.75         3.96    4.80    .416 Wea         
.416 Wea         3.96    4.80         3.97    4.98    .333 Jeff        
.257 Rob         4.26    4.81         4.18    4.75    .225 Win         
.22-250          4.49    4.93         4.19    5.18     6.5x55          
.333 Jeff        3.97    4.98         4.22    5.07     360 No.2        
.17 Rem          4.67    5.03         4.25    5.04    .280 Rem         
.280 Rem         4.25    5.04         4.26    4.81    .257 Rob         
 360 No.2        4.22    5.07         4.39    5.24    .340 Wea         
 6.5x55          4.19    5.18         4.49    4.93    .22-250          
.340 Wea         4.39    5.24         4.54    5.34    .270 Win         
.270 Win         4.54    5.34         4.64    5.39    .300 H&H         
.243 Win         4.68    5.35         4.67    5.03    .17 Rem          
.220 Swift       4.90    5.38         4.68    5.35    .243 Win         
.300 H&H         4.64    5.39         4.80    5.52    .300 Win         
.300 Win         4.80    5.52         4.81    5.56     8 Rem Mag       
 8 Rem Mag       4.81    5.56         4.86    5.62    .30 Fl.Mag.      
.30 Fl.Mag.      4.86    5.62         4.90    5.38    .220 Swift       
.378 Wea         4.95    5.69         4.95    5.70    .240 Fl.NE       
.240 Fl.NE       4.95    5.70         4.95    5.69    .378 Wea         
 6.5 Rem Mag     5.07    5.70         5.07    5.70     6.5 Rem Mag     
.25-06 Rem       5.12    5.88         5.12    5.88    .25-06 Rem       
.300 Wea         5.34    6.08         5.33    6.12     7 Rem Mag       
 7 Rem Mag       5.33    6.12         5.33    6.15     7 Wea Mag       
 7 Wea Mag       5.33    6.15         5.34    6.08    .300 Wea         
.240 Wea         5.64    6.20         5.54    6.29    .270 Wea         
.270 Wea         5.54    6.29         5.64    6.20    .240 Wea         
.264 Win Mag     6.02    6.86         5.81    7.34    .50 BMG          
.257 Wea         6.51    7.17         6.02    6.86    .264 Win Mag     
.50 BMG          5.81    7.34         6.51    7.17    .257 Wea         

01/2003 - 10/2013