Home Forums Hunting & Shooting Ask The Gunwriters Using a chronometer to guess pressures given short barrels or odd bullets.

As I understand it assuming other things equal I can hope my pressures match book loads - that is when the components are the same (within limits of lots) and the firearm is as close as reasonably possible then similar results over the chronograph imply similar pressures.

But when my barrel length is considerably different or my bullet is not listed in data - say .376 Steyr in the Cooper Dragoon where data for the short barrel and for bullets of choice is hard to come by - can I adjust for the shorter barrel at least with listed bullets? If so then how might I choose another bullet and then estimate pressures?

Seems to me that's getting back to the sort of variation that comes with estimating by case expansion.

I have a full set of Handloader (pre 1980 in dead storage though) and such if anyone can suggest specific references.

Using a chronograph to guesstamate pressure is a recipe for disaster, however it can certainly be a factor or guide of sorts.

Pressure is best arrived at by those of us who do not have a million dollars worth of equipment by measureing case expansion at the head of the case, then compare that to what the chronograph gives you and you have a fairly accurate guide to load by..

Actually over the years I can pretty much get er done by simply looking at a case..cratered primers, overly flat primers, are clues ( and only clues) extractor marks are a good indicater to back of one or two grains, sticky bolts are another accurate indicater to back off and usually come after the extractor mark, so keep that in mind..

Using all of the above together has historically been a pretty darn accurate barometer of where we are pressure wise, and the chronograph will give us comparable velocities to compare with the results. Once we are there, then barrel length has been clarified and we know where we need to be at those velocities.

One of the best statements, maybe the very best on the web, on the general topic of expansion and pressure is on this board at:
http://www.24hourcampfire.com/ubbthreads/ubbthreads.php?ubb=showflat&Number=19112#Post19112

Time was I was setup to measure case expansion pretty well - I could easily measure between 2 points across the case to half a tenth or better and so I had some confidence that my measurements were meaningful to a tenth - although I couldn't be sure that my before and after were between the same 2 corresponding points - in fact I'm sure they weren't if defined as points but in the same general area pretty close to the same points.

I'm quite confident it worked for me on a couple of rifles - a .243 that maxed out halfway up the book every time and a .22-250 that I think had a large to amount to almost an improved chamber and accepted older (old style CUP days) book maximums readily - all the signs including accuracy coincided. Equally I have no faith expansion as a pressure sign worked at all on others.

In my old age I load some cartridges and firearms I find pleasant - carbines in .308 and .376 Steyr, a double in .30-'06 (bought as the ideal takedown for rafting/canoeing/kayaking and climbing access) and handguns in 9x23 and .460 Rowland among others. Pressure tested data using the components and firearms like mine to include chambers and barrel length is hard to come by. I can find useful pressure tested data used in the proverbial Remchester 007 and compared to another Remchester 007 but I don't know how and it may not be possible to use data from a 25 inch test barrel and guess pressures in a true carbine with a morphodite chamber and so it goes.

Maximum pressure is reached in about the first 1/2 inch of bullet travel. Barrel lenght does not have anything to do with pressure, but it will affect velocity.

Take a 28 inch barrel and shoot a 60,000 PSI cartridge in it. Then cut the barrel to 20 inches and shoot an identical cartridge in it. The pressure will be the same, assuming you have pressure testing equipment to compare the two shots with, but the velocity in the shorter barrel will be less.

You can use a chronograph to determine the approximate relative pressure of a series of loads and to recognize when pressures are about to rapidly and significantly increase. It will be a lengthly post. Let me know if that is what you are interested in knowing. PS I am sure you meant chronograph not chronometer although both do measure time.
Do not rely on bolt lift as a pressure indicator especially if you have a "blueprinted" action. Same goes for primer cratering. A tight firing pin and hole will deceive you also. By the time you get extrusion into the ejector you are way over.

Varmint Hunter article on case head expansion method

The simple answer to the question posed is that in most rifles you can figure that you lose or gain ROUGHLY 30 FPS for each inch of barrel. Unfortunately, this is only a rough number.

There are calculators available on the web that try to be more precise than that.

If your barrel is 20 inches long, and the book data were taken in a 24 inch barrel, your MV is going to be somewhere in the neighborhood of 120 FPS slower.

Maybe, maybe not. I have seen short barreled guns chrono than than their longer barreled counter parts on the same day and same ammo.

It does happen. Small variations in chamber dimensions can produce more or less pressure, and hence faster or slower bullets. It's not unusual for this to produce a larger effect than a couple of inches of barrel length.

Sorry to disagree with some here, but a chronograph is a far more accurate method of guessing pressure than case-head measurement--and especially primer appearance, bolt lift, etc.

I have run numerous experiments on this over the past decade, with the help of more than one professional pressure lab. If you are using the same powder and bullet as one of the loading manuals, AND that manual's data is from the actual pressure barrel used, then if you are getting about the same velocity, then your loads are in the same pressure range.

The trick here is to use the same bullet, and to find data shot from a pressure barrel. Some manuals develop their data in a pressure barrel, then shoot the loads for published velocities in a sporting rifle. Why they do this I don't know, but some do. Read the manual fine print.

The problems with using case-head expansion occur for two reasons: Accuracy of measurement, and variations in brass hardness. The first is something only you can control, but the second is out of our hands. As an example, I once was running a case-head experiment with the .264 Winchester. Expansion of brand-new brass from two different manufacturers, using the same load from a manual, was .004 and .02 inch. The decimal place is not a mistake. So we cannot predict pressures by measuring brass alone.

The only way we can get into the same ballpark with brass measurement is to measure the head expansion with a KNOWN load and new brass. Then we can then approximate that pressure by using the same make and lot number of brass. But why bother, if a chronograph is just as accurate, and usually more so?

I also have experimented with traditional pressure signs such as primer appearance, bolt lift, etc. These were probably more valid in the days when rifles weren't as precisely made as many today, especially custom rifles. But today's rifles (and brass) often allow us to work up loads far beyond 65,000 psi before any such visible signs of excessive pressure occur. Firing pin fit, squareness of lugs and lug recesses, smooth chambers, etc. all have something to do with this.

An example is the 7mm STW. When Layne Simpson worked up loads for his wildcat in custom rifles, he usually ended up starting 140-grain bullets at 3500+ fps with none of the so-called "pressure signs." But when Remington turned the 7mm STW into a factory round, they pressure-tested Layne's favorite loads and found they were mostly developing pressures of around 70,000 psi, and sometimes more. This is more pressure than most actions can withstand on a sustained basis--and why factory 140-grain 7mm STW ammo usually is listed at 3400 fps, AT MOST.

In all these tests (which have been published in various magazines) I found a chronograph a much better predictor of safe pressures than any other tool available to the average handloader. That said, how much velocity to add or deduct for different barrel lengths varies with individual barrels. But generally 25-30 fps per inch will indeed put you in the right ballpark.

Mule Deer:

The only place I would disagree with what you wrote is when a different powder is used other than that listed in the manual for a given velocity.

This is something that is seldom if ever mentioned, but faster powders can give the same velocities as slow powders, but the pressures can be out of sight with the fast powder, and normal for the slow powder, and velocities still be the same.

As an example, I can get the same velocity from a 7 M/M magnum with IMR 4895 as I can with IMR 4350, but if this velocity is about 3000 to 3100 FPS with a 140 grain bullet, the the loads with IMR4895 have to be at much higher pressures.

Just because your chronograph reading matches the loading manual data does not mean the load is safe.

You might think, who would try to get 3100 FPS from a 140 grain bullet and IMR 4895?

Well, plenty of people who not familiar with the different
purposes and burning properties of different powders.

Using the methods you describe, a chronograph should keep you out of trouble, as long as the chronograph and loading data are used correctly and as each was meant to be used. To use guidelines from your chronograph, you have to use the correct guidelines for powder recommendations.

I bet you already knew this didn't you? I wrote it because I would also bet that there are a lot of re-loaders who think one powder will work in any cartridge just as well as any other powder and think, if my chronograph says it is okay, then the load is okay.

I see a lot of questions on this site about what is a good load for such and such a cartridge with X bullet and Y powder. As far as I know, none of us have access to secret loading data that is not available from the loading manual publishers and powder manufacturers.

I mentioned in paragraph 2 of my post that the same powder has to be used as listed in the manual. But your warning to other handloaders is definitely something that canot be emphasized too much.

I keep popping up with my greatly uneducated idea that loading manuals would be much more useful if they approached load data in terms of� safely� attainable speed with a given combination of components.� Another valuable bit of information (somewhat similar to burn rate charts for powders) would be a chart that rates different brands and models of bullets by their "stickiness" (their tendency to elevate pressure).� The same information in terms of primers and brass would also be nice to know.� (Stan Watson did some work with this in his loading manual for the .30-06.� It is very interesting.) It seems to me that indicating a reasonable speed attainable is of much more value than DO NOT EXCEED loads which may lead peope astray in either direction. Please understand that my goal is not to turn a mouse into a moose by stuffing more powder into it than is prudent. I am merely trying to come up with a workable system for achieving speeds that are reasonable industry standards.��Best to all, John

MD, I think you and I are in violent agreement on most of your points.

In fact, I would go one step farther: A chronograph is a much more precise system than the piezo pressure system. For a given temperature, powder, bullet, and rifle, peak pressure and MV are very highly correlated. For a given load and pressure barrel, probably the absolute most precise pressure estimates we can get with current instrumentation would come from regressing a large number of piezo measurements against chrono measurements, and then relying on the MV measurements from then on. This approach is in harmony with NIST's recommended measurement procedures.

The place I think we part company a little bit is that generalizing chrono based pressure measurements from test barrels to every rifle is tricky business. Different lots of the same powder have different burn rates and will produce different peak pressures for the same MV. While I haven't investigated how much variation that is (yet!), for the time being I treat generalized estimates of pressure based on chrono measurements with some skepticism. But not nearly as much skepticism as PRE and CHE based estimates.

I had two lots of RL22 that produced about 2500 PSI different pressures with the same 30-06 load. Shoulda done the experiment to see if adjusting the load for the same MV produced the same pressure. Didn't. Lost the opportunity.

Not sure why you feel that the current reloading manuels are falling short. I thought that was/is exactly what they are doing

+1

jwp - I guess I just get frustrated with all of the unknowns and unmeasurables in reloading. Since there is no reliable method of measuring pressure available to me, I would like to be told that I can take this 180 grain bullet and that powder in this case with that primer and work up until I have it going 2700 fps in my .30-06. Best, John

5sdad...

Sometimes we get a little bit wrapped around the axle, trying to control everything and make sure we are doing everything right. That's a good thing, because we don't want to lose any friends to firearms that disassemble themselves. Perhaps the following will ease your mind

1,000 PSI more or less doesn't matter much. That is about as precisely as a pressure lab can repeat a 10-shot average, using the same test equipment and ammo from the same test lot. It's about as accurately as we can estimate pressure with piezo equipment.

Your rifle probably has abundant design margin in it. 99.9% of the time, you don't need that margin. If you ever get a case head separation, you'll be darn glad you had all of it. SAAMI pressures are by and large the economical operating point, not the brink of immediate disaster.

Most loads are developed in test barrels (NOTE MD's WARNING!!) which are tight chambered, and which produce higher pressures and MV's than average rifles. Most published loads have that safety margin built into them.

The SAAMI MAP pressure we adhere to is NOT the maximum spec limit for the firearm. The real spec is the MPLM. MAP properly recognizes that in real life, we estimate pressure based on a sample, and samples have a standard error. MAP is set lower than MPLM (IIRC) by two standard errors. It's yet another safety margin built in.

If you're using the powder, bullet, and COL specified in a reloading manual and getting muzzle velocities at or below the published muzzle velocity associated with that load, you are in safe territory.

If you're getting MV's significantly higher than "book", you do not have a fast barrel. You have a small chamber, a hot batch of powder, or a bullet that's harder to engrave and you are not in safe territory.

If you are relying on PRE, CHE, bolt lift, or primer shape to estimate pressure, you are braver than I am.

Denton
I liked that post. Well said.

Royce

Denton,

Yeah, one of the big problems in handloading is variation between productionb lots of the "same" powder. There have been some large variations in certain powders over the past couple of decades.

I would still hold that, in general, muzzle velocity is probably the safest indicator available to the home handloader.

Denton, thank you very much. I like your idea about "getting wrapped around the axle" a great deal. I strive to be safe in my loading - especially so since I am now the mentor to my adult son who has been bitten by the shooting bug. As such, I manage to scare myself away from working my way up to a manual's listed max, even when I have not come near their speed a grain or two below it. Your comments help a great deal. (Again, please note that I am not taking it as license to drive recklessly.) I still feel that there is merit to my notion of a listing of bullets as to their effect on pressure when compared to other bullets. It would, of course, not be an absolute rating, but would, like burn rate charts, give us some idea of what to expect when beginning with a new bullet. Thanks again for your time and effort. It is much appreciated. Best, John