A Powley Computer for Handloaders
Before using these Computers, you should understand their limitations, and
these are considered in the Notes.
- Pointing to marked text
pops up a brief explanation.
- Tab between input fields. Calculations are updated with each new entry.
- Load Computer charges are only for rifles capable of at least 52,000 CUP.
- Use only with conventional jacketed bullets over standard primers.
- Both Computers can underestimate pressure.
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
For external ballistics (trajectory, twist rates, etc), try the calculators at
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
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
Follow the link at this label to comments in the Notes on how to interpret
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
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
, 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
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
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.
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
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)
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
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
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
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
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.