Anybody else heard of this "rule" of reloading?

[taco]

Many years ago (about 18 years ago) when I started getting interested in reloading I was “taken under the wings” of a very knowledgeable and experienced reloader. He used to let me come to his warehouse (he used to rent a small warehouse in the industrial area for his hobby) on weekends to learn the safety and basics of casting bullets and reloading. I spent months going there and learning until he passed away. During one of by visits there he told me that every “professional” reloader knows that for any give load of “factory pressure” (I think he meant “standard pressure”):

* Every 5% increase in powder charge will increase the velocity by 2 ½% and increase pressure by 10%.

* Every 5% decrease in powder charge will decrease the velocity by 2 ½% and decrease pressure by 10%

* Every 5% increase in internal capacity (by seating bullet further out) will decrease velocity by 2 ½% and decrease pressure by 10%.

* Every 5% decrease in internal capacity (by seating bullet deeper) will increase the velocity by 2 ½% and increase the pressure by 10%

I don’t know if this true or not but it made since to me at the time and I still use it as a guideline when reloading.

Anybody else ever heard of this?

[Mal H]

I've never heard it before. With all due respect to the old timer, I don't think there could be any basis in fact to support it. There are far too many variables involved to make such a general statement of fact. To see if it is true for yourself, take any reloading manual and check the figures. One particular aspect that can't be true is pressure. There is a point for almost any cartridge at which adding any more powder will cause the pressure to rise asymptotically instead of in a straight line function as that "forumla" suggests. Although for most calibers, fortunately you'll run out of case capacity before you hit that point. But, if you switch to a faster burning powder, you might not.

I don't think it is wise to use such a general formula in reloading, it could eventually get you in trouble.

[C. D. Beaver]

In his well-written handloader's manual, "Modern Reloading," Richard Lee gives almost those exact figures. In a chapter titled, "An Extremely Interesting Reprint," Lee cites the Vihtavuori
Powder Company and publishes an "inner ballistics" table that compares variables and changes in loads, velocity and pressure.

Such variables as cartridge case volume, bullet seating depth, temperature and overall crtridge length can have major impact on velocity and ballistic performance.

Get a copy of Lee's book, if you can, or make a copy of pages 105-109. Interesting--and informative--reading.





Pro Patria

[Hal]

* Every 5% decrease in internal capacity (by seating bullet deeper) will increase the velocity by 2 ½% and increase the pressure by 10%

That one could get you in a lot of trouble in a hurry. Check out the figures here: http://greent.com/40Page/ammo/40/180gr.htm
and compare them with calculated results using the above 5% formula.

[Art Eatman]

Without getting into the arguments about specific accuracy of the figures, one thing stands out: For a small change in case volume or powder charge, there is a lesser change in velocity and a much greater change in pressure.

It means that when working up a load toward near-maximum pressures, sometimes the gain may not be worth the pain.

Art

[Mal H]

Art, that statement is noteworthy. If someone takes the "rules" to mean that and only that, then it is worthwhile. But if someone uses them as a guide to reloading, then that is foolhardy at best IMNSHO.

However, since the "rules" go down in capacity/vel/pressure as well as up, it would appear that they are meant to be a guide. Not a good thing, no matter what Richard Lee says.

[cheygriz]

In my nearly 40 years of reloading, I would say that your friend's "rule" was close, but not exact. If you think of this as a general guideline and not a "rule" it will be helpful.

As has been mentioned before, when you get close to max, pressures excursions get really unpredictable. A good " rule of thumb," in my experience: When working up "max" loads, follow the procedures in the Lyman manual.

I believe that his "rule" is very close MOST of the time, but please do not use it as a substitute for published reloading data.

I have a personal "rule" that I use for working up max loads. If I can't reload the case more than 3 times, my load is at or near max. If I can reload the case 5 or more times, my load is moderate.

I work up slowly, until I see the first signs of max pressure, then I REDUCE THE LOAD BY 5 PERCENT and consider that MY PERSONAL maximum.

I don't know if this is the best procedure for anyone else or not, but it has kept me out of trouble for nearly 40 years.

Accuracy, power, economy and ease of reloading are desirable. Safety is mandatory.

[Jim Watson]

There have been rules of thumb like this for many years. Back in the 1960s when Hutton, Forker, Forster, and Powley were reporting real technical stuff in Guns & Ammo, the usual estimate was that a small change in powder charge would give an equal percentage change in velocity and double that percentage change in chamber pressure, so 5% more/less powder = 5% more/less velocity = 10% more/less pressure.

The Vihtavouri data is 10% more/less powder = 8% more/less velocity = 20% more/less pressure.

Taco's guy is even more conservative, with 5% more powder giving 10% more pressure but only 2.5% more velocity.

There was one old loading manual, before pressure test guns were common outside the major manufacturers' labs, an old Speer, maybe, that stated that maximum loads were reached by increasing the load until one or another of several signs of excess pressure were seen. Then the load was reduced by 6% for publication. The above rules of thumb say that would give a 12% reduction in pressure, bringing a 60,000 CUP load that gave hard extraction down to 52,800.

There is one writer now who figures that a given percentage change in case capacity as with an Improved wildcat, will give only 1/4 that much increase in velocity, even if optimum powder type and charge are picked.

The problem with such "rules" is that first, they are only good for small changes, I think Vihtavouri extending it out to 10% change is going too far. There is a rule of thumb in geometry, the sine of an angle is APPROXIMATELY equal to the angle (given in radians, not degrees) FOR SMALL ANGLES, less than 5 degrees.

And second, they are best used for interpolation between two known data points. If you try to extrapolate into new territory you have no way of knowing whether the rule will hold true or if you are getting into a range where the powder's burning characteristics are different. I have known people to use one of these rules of thumb to figure their own +P and +P+ loads. This is not smart.

You just have to be careful at the hobby handloading level.

I recall that the .40 cal OAL (case volume) table referred to by RAE and originally shown in American Handgunner was itself calculated by some formula or another and is not the result of actual testing. So it might be right or it might be wrong, and if wrong it might be high or low but you cannot know because it was an extrapolation beyond actual test data.

[Johnny Guest]

- - - At least to a limited extent. Please believe: No flame or disrespect is intended to either view point.

I don't have enough technical background, and certainly not enough instrumentation, to conduct experimental verification or refutation of the old timer's rules of thumb. As stated, there are a huge number of variables, and changing just one or two factors juggles the entire equation. I am therefore uncomfortable with terms such as "EVERY" and "WILL" when referring to rifle and handgun loading. It is easily demonstrated that after a certain point, an increase of powder in a cartridge reaches a point of diminishing returns, as Mal wrote. That pressure rises out of proportion to the benefit gained. But this is is NOT a linear thing, and it is impossible to generalize that with ALL cartridges, an increase of BLANK percent of variable a yields x percent cent change. I do believe there is a good basis for the generalization, though not the specifics.

Best,
Johnny

[WESHOOT2]

1) Safety glasses are mandatory.

2) When in doubt throw it out.

3) ".....but not always....."

4) "From MY gun....."

5) Don't guess; confirm.


Formulas, while interesting intellectually, are (had 'worthless' written here first) not to be counted on; see rules 3, 4, and 5.

[Mal H]

Tim, your "rules" are about the only ones in the reloading game where I can use the words "exactly right" to describe them.

[WESHOOT2]

Thank you.

I must admit that I learned these rules the hard way, except safety glasses.

[Mike Irwin]

Sounds like a good "rule" for blowing up your gun.

As Mal and I discussed as we were heading to the gun show this afternoon, there's a point at which pressure won't rise uniformly, it will SPIKE, often with quite dramatic results.

[9x45]

First of all, pressure is not a linear response. So a small increase in charge weight or OAL may result in a large pressure increase especially with fast powders and heavier bullets. Best bet is to start 10% below the powder makers published loads using maximum OAL and chrono your loads (same brass headstamp for same case volume). Depending on what power floor you are trying to make, USPSPA or IDPA, then find the most accurate load for your gun.

[WESHOOT2]

I like it most when I add more powder and the bullet goes slower.

Yeah, that's MY favorite.

[taco]

I don't believe he meant I should follow this "rule" to develop my own magnum loads but to poinout the relationship between velocity and pressure. I wish I knew a little more at the time to ask more questions.

I believe the point of his "rule" is that price to pay for little extra velocity is a whole lot of pressure. Also, the point of internal capacity and pressure just points out how important OAL really is when reloading.

Again, I believe this is a very general rules to follow if only to keep you aware of correlation between velocity and powder/capacity. It has made me think twice before reloading anything "warm" and I have learn to be very careful about powder charge and OAL of my loads.

One more thing, I believe the word I should have used is "For each 5% increase..." instead of "Every 5% increase...".

[Desert Pete]

I asymptotically agree with the old timer. I like that word. I'll use it every asymptotical time I can.

[Steve Smith]

Don't put some much Varget in that the primer backs out when you seat the bullet!

[Art Eatman]

I started out reloading when Phil Sharpe's "Complete Guide to Handloading" had numerous useful loads for me and my .30-'06. There just weren't many reloading manuals, compared to today's world. Today, not only are there more powders, there's more info about how they perform and why they do what they do.

For whatever reason, I just never got interested in trying to "beat the system". And that's what meddling around with any charge greater than a book-max really is.

Now, book-max, to me, is a for-sure limit for a tight chamber. It's real easy to measure a fired case and find out if your chamber is a minimum or maximum. If it's a bit over minimum, just relax and enjoy it; you have a bit of wiggle-room, safety-wise.

And Bambi will never know the difference between a safe 3,000 ft/sec and an unsafe 3,050 or 3,100.

Reading the gun rags for some 50+ years, I note that accuracy loads are almost always a bit under max. It's very rare that the smallest group in some test comes from a max-velocity load.

FWIW, Art

[cheygriz]

And Bambi will never know the difference between a safe 3,000 ft/sec and an unsafe 3,050 or 3,100.

Reading the gun rags for some 50+ years, I note that accuracy loads are almost always a bit under max. It's very rare that the smallest group in some test comes from a max-velocity load.
FWIW, Art
==============================================

Well said, Art, very well said!

[Ackley Improved User]

The answer is easy - get the NECO internal ballistics progam (see relevant thread) and answer the question for yourselves. The data is available for your individual gun, caliber, powder, and bullet.

[Dave R]

It's very rare that the smallest group in some test comes from a max-velocity load.

But it can happen. I have never gone over published max, but the load that is most accurate in my .308 with the 150gr. Speer Hot-Cor is the published max for H335.

[Leftoverdj]

Being an old guy, I agree with the old guy, within the working limits of the powder, anyway. As someone already pointed out, some fast powders can go plumb hysterical at a small increase when they are already at the top of their working range.

Aside from that exception, many of us use the general principles stated, often without knowing we are doing so. If you shoot an AI or a wildcat cartridge, you are taking advantage of the increased capacity to use a more powder to get a higher velocity at the same pressure. Same story with my 3" .257 Roberts. Same story in reverse with a .35 Whelen I had with a stock Mauser magazine. That rifle would take full book max loads with 250 grain RNs, but the load had to be cut a grain and a half with the same weight spitzers. They had to be seated a bit deeper, reducing useful capacity and increasing pressure.