Chronographs and Pressure

One tool often advocated for the handloader is the chronograph. It is put forth one can keep a lid on chamber pressure by watching velocity.

At the surface, this seems foolish, and a couple of simplistic arguments quickly come to mind. Certainly, one cannot work up a load with 4227 that will duplicate the performance of a load which is optimum with 4350. A bore obstruction results in no velocity and pressures which can burst a barrel.

Those, of course, are extreme examples, so might the chronograph provide a gauge of pressure for normal conditions? Homer Powley hoped to do so with his Powley Computer. In preparing my version of the Powley Computer, I studied this problem, and I'll give here the numbers I came across. My conclusion is you can't rely on a chronograph to keep you safe.

To begin, one needs data on how pressure and velocity vary as various components of a rifle cartridge are changed. The figure of merit is how fast pressure rises for a given change in velocity. One can state this as a ratio such as 3:1, or "3 to 1," meaning pressure rises 3% for every 1% rise in velocity.

Changing Charge

This is the most important ratio to study, since it is by adjusting charge that one is supposed to adjust the velocity seen on the chronograph. Modern reloading data, such as that from the web sites of Hodgdon and IMR and reloading manuals such as Lyman's 48th and A-Square's all include pressures and velocity for max and starting loads (and sometimes in between as well).

I considered data from all 4 sources and limited the choices to those tested in psi, not CUP. Powders included spherical and single and double base stick. The pressure rise rates, compared to fps, run from about 1.5 to 4.0, although it went over 8 with one low pressure cartridge at low loading densities.

Overall, one can say that pressure typically rises about 3 times as fast as velocity, when adjusting charge.

Changing Primers
J. Barsness tested the effects of primers on velocity, and gave this information, for a .300 Win Mag (I failed to record the powder and the bullet weight reported in the 06/2004 issue of Handloader):
                fps     psi

WLRM            2991    70100
Fed 215 M       2964    63800
CCI BR2         2920    55800
Pressure rose nearly 26% and velocity over 2%, for a ratio of 11:1.

Any Shot You Want (A-Square's manual) has similar data, for the 7 Rem Mag using H4831 and a 160 gn bullet:

                fps     psi

Win WLRM        3045    67600
Win WLR         3024    64400
Fed 215         3036    61400
CCI 250         3039    61500
Rem 9.5M        3041    59300
CCI 200         3011    54800

Here, the pressure rise was 23%, and the ratio of the percentage changes is a whopping 20:1. This doesn't mean, though, that if the charge were increased so that the pressure with CCI 200 primers was 62 ksi, that changing to WLRM would still increase pressure 23%, to 76 ksi. I have no data regarding this, and I believe it would not. However, were one loading for an older, medium pressure case, such as H&H's .30 Flanged Magnum, changing primers clearly can turn a warm load into a proof load.

For smaller cases, such as the .44 Rem Mag, large changes have also been seen. C. Petty reported on the effects of changing primers and powders in the 12/1997 issue of Handloader magazine. His numbers can be summarized simply: a 63% rise in pressure might be accompanied by only a 7% rise in velocity, a ratio of 9:1.

Less alarming is the data from Lyman's 48th Reloading Handbook, for the .308 Win:

                fps     psi

CCI 34          2545    55400
Win WLR         2539    54700
CCI 200         2526    54000
Fed 210         2508    53100
Rem 9.5         2509    52800

The pressure rise here is 5%, and the ratio is less than 4:1. (Lyman also noted shot to shot variations can be over 9 ksi in some loads and that switching primers can drop this to well below 4 ksi. The effect on velocity was not mentioned.)

In summary, primers can cause drastic changes in pressure, with the ratio of pressure change to velocity change running about 10:1.

Another summary regarding primer substitutions was found in an article by M.L. McPherson, who has edited several books on shooting including Cartridges of the World and the manual to the internal ballistics simulator QuickLOAD:

primer substitutions have been proven to sometimes radically alter pressure--this is not unique, ballisticians tell horror stories of customers who haphazardly substituted primers and ran into king-sized problems--for example, combining a milder than suggested primer with Accurate #2 in a 9mm load and doubling pressure. Moreover, neither you nor anyone else can reliably predict results of any primer substitution. Use of a "hotter" primer can result in lower pressure and use of a "milder" primer can result in higher pressure--any result is possible and all have been observed.
which echoes his comments found in the User's Guide to QuickLOAD:
Results of primer substitutions are thoroughly unpredictable. For example, use of a hotter primer is apt to raise pressure and velocity but it can produce the opposite result; worse, it might increase pressure and reduce velocity or decrease pressure and increase velocity! There are reasonable explanations for each of these potentialities.
Obviously, a chronograph isn't going to help you sort out these issues.
Changing Bullets

Many handloaders assume you can use any bullet of equal weight in any recipe found in a loadbook. I made this mistake myself, early on. The NRA's Handloading (1981) has some older CUP data showing the effect changing bullets can have on pressure. This data was for a .30-06 with 150 gn bullets over the same charge of 3031, and for all bullets, the seating depth was the same:

                fps     CUP

Spr RN          3037    51900
Win ST          3031    50700
Sie SP          3021    50100
Nos Part        3018    48200
Hdy SP          2999    48200
Hdy RN          2972    47400
Spr SP          3000    46500
WTCW            2967    45400
Bar SP          2990    45400
US FMJ          2956    44500
The pressure rise is 17% and the ratio of the rises is 6:1. More than bullet construction would have varied in this test; bullet jump had to vary as well.

Any Shot You Want also has data, for the .308 Win with 180 gn bullets:

                fps     psi

Sie BT          2610    53200
Sie RN          2557    50100
Sie HPBT        2611    49800
Hdy SP          2591    48900
Nos BT          2567    47700
Nos Part        2574    47100
They did not state if they kept seating depth (and net case volume) the same or if they kept COL the same. Note the highest velocity came at pressures below the lowest velocity. Discarding the RN data, pressure rise is 13% and the ratio of pressure and velocity rises is 8:1.

One might conclude that changes in bullet construction will cause pressure and fps to move in the ratio of about 7:1.

Changing Powder

Different production lots of powder can vary remarkably in burning speed, or quickness, and Any Shot You Want has numbers for this as well. With (then) recent lots of 4064, they found fairly consistent pressures and velocities, and while the spreads were small, one can still compute a ratio; it is was nearly 9:1. They also had indirect data (the case brand was also different) for two different lots of H4831, with one producing 7% higher pressure but 4% lower velocity.

To quote A-Square, "our propellant assessments in preparation for making runs of our ammunition over the past years shows significant differences from lot to lot from all manufacturers."

Changing the Case

Different cases can have different internal volumes. The smaller the volume, the higher the pressure generated, all other factors being kept the same. Each 8.5 gn increase in case weight reduces net case capacity about 1 gn of water.

Cases can also vary a bit in neck thickness, and this will affect the grip of the case on the bullet. Bullet pull in turn can change the initial rate of pressure rise in the cartridge.

Data for a .30-06 with a 180 gn bullet is found in the A-Square manual:

                fps     psi

Fed             2718    57200
Rem             2709    55800
R-P Nickel      2708    54800
Win             2680    52900

The rise in pressure is 8% or six times that of velocity. They also had samples of 50 year old Winchester .30-06 brass (which was likely thinner than modern brass). Keeping all else the same, they found the modern brass raised psi 10% or four times as fast as fps.

Shot to Shot Variations

Lyman's 47th offers typical variations in a 10 shot string:

                fps     psi

 1              2603    50400
 2              2605    51000
 3              2637    53000
 4              2601    51000
 5              2630    52500
 6              2620    52300
 7              2627    51900
 8              2634    52500
 9              2660    55300
10              2639    52600

This is a nearly 10% variation in pressure and more than 2% in velocity, a ratio of more than 4:1.

For their load listings, the A-Square manual gives the extreme spread observed in both velocity and pressure over several rounds with that load. Assuming the spreads are centered on the averages they report, one can compute a rise ratio, but this is a big assumption. However, I ran a few numbers and they were typically about 7:1.

An extreme example of what can go wrong in load development was given in the 06/1997 issue of Handloader where a combination of components destroyed the test rifle. The barrel was refit for piezo testing, and from the sensor traces, it was decided the primer blast caused the bullet to jump the leade and become lodged in the throat, forming a minor bore obstruction once pressure recovered.

                fps     psi

1               2601    48800
2               2662    53800
3               2708    57600
4               2720    58000
5               2687    54100
6               2731    58600
7               2754    62200
8               2875    82100

Pressure rises about 6.5 times as fast as velocity. Note how pressure rose from the mid-50's to the low-80's, exceeding proof pressures. If nothing else, this example shows why loads with large fps swings on the chronograph should be discarded.

The data for the shot-to-shot variations reinforce the other data given above, namely, minor variations which affect either initial bullet acceleration or charge ignition will cause pressure and velocity to move in a ratio of about 6:1.

Conclusions

The numbers suggest to me one can get into trouble by relying on chronographed velocity to gauge pressures. If the relationship between pressure and velocity when adjusting charge is about 3:1 (but can be less), how can one expect charge adjustments to correct for component substitutions which can cause pressure and velocity to move in the ratio of 6:1, 10:1, or more?

We could use the primer substitution example given in the A-Square manual and assume the effects compound (but see below). Here, a "magnum" primer was substituted in a "magnum" cartridge. This raised pressure 23% and velocity a bit over 1%. The handloader, on seeing the fps change on his chronograph, reduces the charge to get the velocity he expects. Data from A-Square shows pressure dropping about 3% for each 1% drop in fps. The primer raised pressure 23% (or 1.23) and dropping charge might lower that by 3% (or 0.97) for a net change of (1.23 * 0.97) = 1.19, or 19% beyond where he thought he was. That's 2/3 the way to a proof load.

The primer example is, of course, an extreme one. If one avoids specialty primers such as the WLRM, most component changes cause pressure to move at about 6:1 over velocity. Say pressure rises 12% with a velocity change of 2%. Compensating by dropping the charge should drop pressure about 6%, for a net rise in pressure of 5%. While not ideal, you should be safe at such pressures.

Unresolved are questions on switching two components in a recipe, say the bullet and the case. Do these pressure changes compound? Unfortunately, I have no data on this. I suspect they compound somewhat, but not fully.

The numbers presented hint a chronograph can help keep a lid on pressure for many but not all loads. The good folks who put together the A-Square manual indicate one might do so:

It once again underscores the necessity to control your loading by use of a chronograph and loading to the expected velocities.
yet the data they give in various parts of their book seem to contradict this advice.

Perhaps the writers of the A-Square book were trying to suggest it is safe to work up to the velocity found in pressure barrels when one is using all the same components tested. By this, I mean so long as the listed case, bullet, powder, and primer are used, one might adjust the charge as needed while seeking to match the pressure barrel velocity. One obviously shouldn't get carried away here. If you've increased the charge weight 5% to try to reach the expected velocity, suspect the chronograph may be off. If you reach the expected fps with far too low a charge, check the chronograph and for something amiss in the barrel or chamber (and try to recall if you used the correct powder). Do keep in mind that any load that reaches book value in your rifle may exceed it in someone else's rifle, so don't give such cartridges to your friends.

Still, I think it best to heed these comments by Ken Oehler, a maker of pressure measuring equipment for ballistics labs:

Loading to get the book velocities is no guarantee regarding pressures. Getting the book velocities might tend to imply that you had close to book pressures, but there is no guarantee. You must have enough pressure to get the velocity, but getting the expected velocity does not mean the pressures behaved. On the other hand, exceeding book velocities is a guarantee that you exceeded book pressures.

If all velocities followed the books, there would be no need for chronographs. If all pressures followed velocities, there would be no need for measuring pressure. We are playing a game where there are many variables; we recognize some and can even measure a few of them.

One other conclusion relates to internal ballistics software such as QuickLOAD and the Powley Computer. These do not account for primers, etc., so their predictions for fps must be used with caution.

Suggestions

Drop your velocity expectations by 5%. This gives no guarentee, but it stacks the odds in your favor. Using the 3:1 ratio typical when adjusting charge, this gives a pressure drop of 15%, a decent margin.

One thing to keep in mind with chronographs is their accuracy is not perfect. Barsness has played with numerous chronographs over the years, and he reported, in the 02/2002 issue of Handloader, that the less expensive chronographs can be 25 fps off the speeds shown by the best quality units. So, dropping an additional 25 fps off any velocity you want to attain would be a good idea, unless you're using an Oehler 35, the chronograph generally regarded as the best available. With the Oehler 35 temporarily off the market, you might consider using two different, cheaper chronographs, such as one ProChrono and one Chrony, mounted back to back on a single sawhorse. Other makers of chronographs are CED and PACT.

However ...

... a definitive test has not been done. Hopefully an enterprising gun writer with access to a piezo bench, such as Barsness, will try to compensate for a hot primer by adjusting charge weight, and he may demonstrate a trend not obvious. For instance, the remarkable pressure rise reported with primer substitution may be unique to cases large relative to the bore, or it may be the effect dimishes as pressures rise. Perhaps there will be another edition of the A-Square manual, or the next Lyman, which might address this topic. Another possibility is for a handloader with a strain gauge rig to work through this problem and report his results.

Until then, I'll continue to chronograph to velocities lower than suggested.


11/2005 - 08/2006