Load Computer
(this input column)
Pressure Computer
(both input columns)
(related outputs)
Case Capacity gn Charge gn Seating Depth in
Case Length in Velocity fps Net Capacity gn
Cartridge Length in (output) Bullet Travel in
Bullet Length in Pressure CUP Expansion Ratio
Bullet Weight gn Mass Ratio gn/gn
Bullet Diameter in Sectional Density lb/in/in
Barrel Length in (warnings) Relative Capacity in
(outputs) Quickness
IMR Powder disable help pop-ups Loading Density gn/gn
Charge gn Kinetic Energy ft-lb
Velocity fps Pressure psi Efficiency %
Cartridges: Bullets: Examples:            

A Powley Computer for Handloaders

Before using these Computers, you should understand their limitations, and these are considered in the Notes.


The Powley Computer is an empirical method for predicting charges and performance with the single base IMR powders. For a full simulator—one which covers a greater range of powders—try Kolbe's P-Max. It is available for both smokeless and black powders.

For external ballistics (trajectory, twist rates, etc), try the calculators at JBM Ballistics.



H. S. Powley

Enter the weight of water needed to fill an empty case to the mouth. It is better to measure a fired case, before it is resized.

Approximate case and cartridge dimensions for several common cartridges are available in the Cartridges list, below. Follow the link at this label to an adjoining file listing the case capacities of many other cartridges. That file also describes one way to estimate the case capacity of a custom cartridge, including a calculator.

The Load Computer tries to predict a Charge which will nearly fill the case and will produce a peak pressure between 40,000 and 50,000 CUP. With small bore cartridges, the pressures will likely be on the high end of this range.

Clicking here transfers the Load Computer charge and velocity estimate into the Pressure Computer, as inputs. For powders 3031 and slower, the Pressure estimate will be about 43,500 CUP, and this can be taken to be the target pressure of the Load Computer. For the faster powders, the Pressure estimate is about 40,000 CUP, suggesting Powley wanted a more conservative load with these.

The reference scale for the Quickness calculated by the Load Computer is
180    4227
160    4198
135    3031
120    4064
115    4895
110    4320
100    4350
 95    4831
Follow the link at this label to comments in the Notes on how to interpret such numbers.
This Computer has a limited list of bullet lengths, below. However, if the bullet weight and diameter have been entered and if one of these two buttons is set, the Computer will estimate the length of the bullet. If the left button is set, the length estimated will be that of a spitzer, but if the right button is set, that of a round nose will be. If the button on the other side of the input field is set, no estimation will be made. (Previous versions used checkboxes, but IE could not process these correctly.) The estimations for RN bullets are good, but spitzers vary greatly in length.

Using these bullet length estimations allows one to quickly see how necking down a case affects performance at a given bullet weight. For instance, you can compare a 250 gn RN in the .30-06, .338-06, and .35 Whelen by selecting each from the cartridge window, with no need to go to the bullet window.

The calculator assumes flat base bullets. For a given seating depth, boat tail bullets leave a bit more room (net capacity) in the case. To compensate for a boat tail, one can add about 1 gn to the Case Capacity for 37 caliber bullets, about .5 gn for 30 caliber, and about .1 gn for 22 caliber.

The link at this label points to a basic twist rate calculator.

This is the weight of the Charge (second column) divided by the net case capacity (this column). The Pressure Computer is best used with values near 0.86.
breech to muzzle
base of bullet to muzzle
net case capacity, below the seated bullet
This is the net case capacity (volume) divided by the cross sectional area of the bore. This value characterizes cartridge case capacity just as Sectional Density characterizes bullet weight. Values run from about 0.9 inches for the .30 Carbine to about 4.8 for the .300 Wea to above 6.0 for some small bore cartridges. The predictions of the computers seems to be better with cases around 3.0, which lies between the .308 and the .30-06.

In the Load Computer, powder selection is determined by SD and Relative Capacity (RC); see the pop-up at IMR Powder, in the first column. While Powley stated his equations in terms of Mass Ratio (above), it is easier to visualize RC.

Enter the weight of the charge in the cartridge being tested.
Enter the chronographed velocity of the cartridge being tested. Using an average over several samples is prudent, but see the discussion in the Notes.
This is a rough estimate of the true pressure indicated by the Pressure Computer. It was not suggested by Powley, and no accurate, universal conversion between copper crusher readings (eg., CUP) and true psi exists. The estimation used here is based on (1) the CUP and psi cartridge pressure limits suggested by SAAMI, (2) these limits as suggested by the CIP, and (3) test data (right) from Lloyd Brownell in the 09/1968 Handloader.
This is the ratio of the charge weight to the bullet's weight. Together with Expansion Ratio and Pressure, it determines the cartridge's performance.

If no Pressure Computer inputs are present, the MR is computed from the Load Computer outputs. Otherwise, the charge given to the Pressure Computer is used to compute MR. In the Pressure Computer, MR outside the range 0.2 to 1.0 requires extrapolation from Powley's data.

This is the volume behind the bullet as it exits divided by the cartridge's Net Capacity. Together with Mass Ratio and Pressure, it determines a cartridge's potential. ER greatly affects muzzle pressure and thus muzzle blast. In the Pressure Computer, ER outside the range 5.0 to 13.0 requires extrapolation from Powley's data.
Efficiency is the bullet's Kinetic Energy divided by the chemical energy in the charge. It is computed from the Pressure Computer inputs for charge and velocity. For the calculation, the chemical energy in the charge is estimated to be 185 ft-lb per gn of powder, an average value from QuickLOAD for the IMR powders (however, 178 is given in the NRA Fact Book of 1988).

Efficiency is primarily a function of peak pressure and Expansion Ratio, and increases in either improve efficiency. The efficiency shows why increasing the case capacity doesn't increase the velocity as much as one might expect: "improving" a case decreases the ER.

From cartridge dimensions and chronographed velocity, the Pressure Computer estimates the pressure that would be indicated by copper crushers. While the term "CUP" is properly applied only to crusher tests done to SAAMI procedures, it is used here to represent any radial copper crusher reading.

The Pressure Computer tends to underestimate the pressure for loads around 50,000 CUP, and it also tends to overestimate pressures near 30,000 CUP. See the Notes.

If a value for pressure is entered here and if the Load Computer inputs have already been entered, the computer calculates a charge and velocity which would indicate that pressure. This can be treated as an estimate of cartridge performance at that pressure. No indication of which powder would be used to attain that performance is attempted.

Use the checkbox to hold the Pressure fixed. So set, each change to Load Computer inputs generates a new estimate for performance at that pressure. Because the Computer tends to underestimate higher pressures, it is best to take between 2000 and 4000 CUP off the Pressure limit when near 50,000 CUP. Using 48,000 for performance estimates is a fair limit, but for cartridges combining large Relative Capacity with low SD bullets, use about 43,000—see the Notes.

Follow the link to an adjoining file for a list of cartridge pressure standards, and keep in mind the Powley Computers use "crusher" pressure readings, not piezo.

If the bullet diameter has already been entered, the computer will calculate the bullet weight when a new value for SD is entered. For hunting with bullets of conventional construction, an SD near .300 is often recommended.
Read the discussion on powder quickness in the Notes, and consider the calculated Quickness shown in the third column. In some cases, the Computer can indicate a powder too fast. Be especially wary of predictions for 4227 and 4198. Powder selection in the Load Computer is determined by SD and Relative Capacity (third column) and is mapped below. Follow the link at this label for a further discussion.

Point to a button for comments. Click the button to enter that example's inputs into the Computers. The examples include those in which the Powley Computers give correct predictions and those in which it fails.

The Computers' outputs are compared to published values from reputable load books. (Cartridge dimensions entered into the Load Computer are, of course, approximations.) The charge and velocity from the load book are entered as Pressure Computer inputs.

(click) This is the .30-06 loaded with a 150 gn SP bullet, and it shows the Powley Computer at its best. The numbers can be compared to the pressure tested loads in Lyman's 48th Handbook. Lyman's data shows the charge from the Load Computer would be quite safe, even if the velocity is rather optimistic. Lyman's data shows that maximum performance comes with the 4064/4895 class of powders. The Computer suggests using a slower powder, 4350, which results in somewhat lower pressures, a goal of the Computer. Note that the Quickness value is on the border of the medium and the slow classes of powders. The CUP estimate from the Pressure Computer is calculated from Lyman's maximum load for 4064, and the estimated pressure is within 1000 CUP of the measured 49,600.
This is a .22-250 loaded with a 63 gn bullet, a cartridge which can be found in Lyman's 48th. Here again, the Computer has selected a powder slow enough that the charge weight given does not exceed pressures. Lyman found that 35.0 gn of 4064 was maximum and that a compressed charge of 4350 was needed to get the higher velocities.

The Pressure Computer was given Lyman's max load, of 4064, and the pressure estimate is below the measured 50,800 by nearly 4000 CUP. Greater percentage errors in the pressure estimate can be found, especially with smaller calibers, when one combines low SD bullets with relatively large case capacities; see the Notes.

IMR's site 10/2005 had data for the .358 Win. loaded with a 250 gn spitzer. The Load Computer picks the powder IMR reports giving the best velocity and suggests a charge which is below maximums. The Pressure Computer estimate is 3000 CUP below the lab measurement.
CUP data for the .300 H&H loaded with 220 gn bullets was found on IMR's web site, 08/2005. The Load Computer tries to predict a conservative load, one which when loaded for a nearly full case will give pressures comfortably below maximums. IMR's data shows a nearly full case of 4831 is right at maximum. Since 4831 was the slowest IMR powder available to reloaders when the Powley Computer was developed, the Computer is correct in showing a suitable IMR powder didn't then exist.

IMR's data for 4831 is entered into the Pressure Computer, giving a CUP estimate 6% (3,500) below the 54,000 measured.

Entering the cartridge's maximum of 54,000 CUP into the Pressure returns a velocity estimate of just over 2800 fps. I could find no published load quite that fast, but this was not unexpected; the Pressure Computer tends to underestimate pressures near 50,000 CUP.

One can find .444 Marlin loads for 240 gn bullets in IMR's Guide dated 05/01. While the pressure limit for the .444 is lower than that intended for the Load Computer, the numbers were run to see what would come out. Some sources list a case capacity of 69 gn, and this is used in the example. The Load Computer indicates a charge of 4198 which is reasonable, being below IMR's maximium of 47.0 gn. Entering IMR's maximum data for 4198 into the Pressure Computer produces an estimate which is 9% below the 44,000 CUP measured.

Other sources list the case capacity as 66 gn. Entering even 68 gn for the capacity causes the Computer to indicate 4227 as the powder. Comparing this charge to IMR's 4227 data (where 32.5 gn gave 44,000 CUP) suggests a pressure well in excess of 50,000 CUP would be generated.

Use these Computers only with rifles capable of high pressures and be wary of indicated charges of 4227 and 4198. With these two powders, it would be prudent to begin load development with the next slower powder, ie. 4198 and 3031, respectively. Such problems with the indicated charge seem to be more likely with cartridges combining a bullet of SD below .200 with a case providing a Relative Capacity (third column) below 2.

IMR's site on 08/2005 had data for the .35 Rem loaded with a 200 gn RN bullet. The Load Computer happens to indicate the very load which is maximum. One goal of the Load Computer is a charge producing a peak pressure between 40,000 and 50,000 CUP. Here, only 34,700 CUP was recorded in the lab.

Using a Load Computer charge in a low pressure cartridge such as the .35 Rem is not recommended. The example only shows that Load Computer charges will not always generate the desired pressure, which in turn results in the velocity estimate being off. Note that for a rifle capable of 52,000 CUP, the charge indicated is quite safe, if rather mild.

In this example, the Pressure Computer's estimate of pressure is 2% high, and in general it overestimates pressures near 30,000 CUP.

This is an example of developing a load for an obsolete, lower pressure cartridge. The dimensions for the .33 WCF are entered into the computer. The Pressure Computer is given the claimed factory velocity of 2200 fps (the Computer is fairly insensitive to charge weight) and produces an estimate of about 34,000 CUP, in line with the pressures reported by Whelen back in 1918.

This 2200 fps velocity is divided by the Load Computer's estimate of 2477 (at about 44,000 CUP) and found to be 11% lower. Using a ratio suggested by Powley (see the Notes), the Load Computer charge should be dropped by 11% as well, giving 41.9 gn. Since 4895 is a powder recommended for reduced loads, such a reduction in charge is not unreasonable. (Do not greatly reduce 4350 and slower powders.)

Hornady's 1977 data for the .33 WCF shows 2200 fps was obtained with 45.1 gn of 4895, and starting loads down to 35.1 gn were listed. Another source listed 45 gn of 4064 giving about 2200 (but 45 gn of 4895 giving nearly 2500 fps!). Load development should begin with a charge at least 5% below that computed (or 39.8 gn), and the load should not be worked up beyond 2200 fps on the chronograph; indeed, 2100 fps would be a prudent limit.

based on the velocity from the second column
If a new value for the bullet seating depth is entered and if the case and bullet lengths have also been entered, the computer will calculate the corresponding cartridge length (first column).
The checkbox near the center of the form disables these.
This button brings up a list of cartridges. Selecting a cartridge enters into the Computers values for the case capacity, the case and cartridge lengths, the bullet diameter, and the working pressure. For cartridge lengths, industry maximums are entered, unless this makes seating depth too short with bullets at the SD typically loaded. For pressure, industry maximums are entered unless, as in the case of the .45-70, factory loads are known to be loaded to lower pressures. Know that many factory loadings have lengths and pressures below maximum. Mind the Seating Depth (third column) when trying lower SD bullets.

Caution: in the list, cartridges in dotted outline operate at lower pressures and must not use Load Computer outputs directly; see Example 7. These cartridges can be used to test the Pressure Computer at lower pressures.

Beware: from the Pressure Computer, the predicted performance for cartridges operating near 50,000 CUP is nearly always higher than possible in pressure barrel tests; see the Notes. The predicted performance near 40,000 CUP is generally good, but near 30,000 CUP it tends to be low.

This button brings up a list of bullets of various calibers, weights, and styles. Selecting a bullet enters into the Load Computer values for the bullet length and weight. This version of the Powley Computer assumes flat base bullets, and all bullets in the list are so. Dimensions for Woodleigh bullets were copied from their web site; all other lengths were copied from QuickLOAD's lists.

Note: no check is made that the bullet diameter matches that of the cartridge. The length and weight alone are copied.

For the charge indicated, the velocity estimate is sometimes close, but for some cartridges it will be rather optimistic; see Example 6.

For a cartridge rated near 52,000 CUP, the velocity predicted by the Load Computer can be safely attained, but the powder and charge weight indicated may not do so.