Pressure and Alloy Matching for Accuracy. Fact or Fiction in a handgun?

The engineering on this topic is fun to read about and for discussion. There are BHN charts, lead deformation charts, calculations to provide obturation pressure, accurate-pressure and a way to determine the maximum pressure for cast boolits. However taking it to the range and validating something like "matching pressure to BHN for accuracy" to me reveals the truth of this widely accepted and commonly promoted & quoted “claim”.

The reason I even bring it up is there are plenty of articles and books about this that get read and then recited as fact.   Many people are trying to figure out what’s wrong with their gun because of their bad results at the range.  This concept is often promoted by pundits and then is passed along as fact in various online forums by folks that have assumed it's valid.


But is it?

First, the Context

My goal is the best accuracy possible and all my comments should be read with that in mind.  What I’m about to cover may not/will not apply to someone who has decided they need a max charge of H110 with their 158 or 168 boolit and want to then make it a bit more accurate, keeping it at the max charge.   


I have yet to get the best accuracy with full house loads therefore don’t work with them much, but hope to in the future.  But using a download charge of 10.3 grains of 2400 (168 Keith) is not bad with 1.2 inch groups at 25 yards very possible.  It’s a far cry from a max charge though and was derived by searching for the most accurate 2400 load, not trying to achieve maximum velocity.  


I hope that made the point and helps clarify the goal of this blog. 


What is Pressure and Alloy Matching for Accuracy?

Finding information that describes the approach is pretty easy.  Modern Reloading by Richard Lee is one of the better known sources so I’ll briefly give his formula.  Note that he tested with rifles and not handguns, just like many other proponents.  Richard Lee never claimed this works in handguns, as far as I know so this isn't a slam against him or the book.  From that, many others have assumed it applies to handguns too, they are the ones that promote this as fact:


BHN*1422*.9=Accurate-pressure for cast bullets, called Max pressure in the Lee book, another key formula is:
BHN*1422=Alloy-strength and part of the information distributed by Lee, also;

BHN*1920=Maximum-pressure for cast bullets (this isn't based on the Lee book and is much higher than the Lee alloy strength formula)


Based in these, the accurate-pressure for my BHN 8 alloy is 10,370PSI, the alloy-strength is 11,520PSI and the maximum pressure is 15,360PSI.  Using the same formula for accuracy the best BHN for a 30,000PSI load is roughly BHN 24.


Compare these numbers to the results. 


Testing in a 357 Magnum for Accuracy, Results 


After testing several thousand boolits of different hardness, different diameters, different OAL, with and without gas checks I have proven beyond any shadow of a doubt that, first, one of the most accurate loads with a 358-429, 168 Keith in my 357 magnum is:

A 700X 30,000 PSI  load is most accurate with BHN 7-8 (< 1inch groups using 700X) 


This is 3 times the ideal matched pressure/alloy and is 2 times the maximum pressure.  The results are starting to prove that pressure matching is a hoax, at least in a 357 Magnum revolver.


Now the question is, does lower or higher pressure perform best with correspondingly lower and higher  BHN?  If not, pressure matching isn't for real and is disproved,

The most accurate BHN with other pressures and using HP-38 (slightly bigger groups by .25 to .5 inch bigger at 25 yards, still <1.5inch which is very good) are:

A HP-38 10,000 PSI load is most accurate with BHN 7-8
A HP-38 20,000 PSI load is most accurate with BHN 7-8 (this equals 700X, but isn't as consistent)
A HP-34,000 PSI load is most accurate with BHN 7-8

Any increase in BHN opens the groups and is less accurate for each of these loads. The harder the alloy the bigger the group.  The same BHN at all pressures is the most accurate with the same or similar powders.  That's worth talking about in another post as it turns out to be important.

 

Matching or increasing the alloy BHN with the pressure caused bigger groups.  Accuracy suffered!

I’ve tested a bunch and let me say this isn’t my entire database.  But it is one striking example that I trust makes the point. 


The Conclusion

This completely disproves Pressure and Alloy Matching for Accuracy, in a 357 Magnum Revolver.  It isn’t even helpful when trying to achieve better accuracy.   

Added: I've been told that once an accurate load is worked up, the alloy and pressure is matched.  This interpretation of pressure and alloy matching is limited and mistaken in this context. That's simply the most accurate charge for a given power with that bullet.  

To test this, change to a different powder (especially one faster or slower) and work up the load.  There's a good chance it is most accurate at a different pressure.  

Which pressure is correct for that alloy, according to the formula there is only one, but in practice it changes with the powder used.

Alloy and pressure matching does use peak pressure as the measurement.  That's what is being addressed here.  If someone tries to complicate the issue by pointing out the pressure curves change with a powder change, knowing that fact doesn't make the pressure-alloy matching valid or predictable. It means the most accurate load is at different peak pressures for different powders.  The most accurate powder is most often at a pressure very different than derived by alloy-matching.

It also means the most accurate load for a given powder must be worked up the old fashion way.  By shooting and measuring groups at the range.  Not by using pressure and alloy matching formulas.

Does matching work in other handguns?  I’d be speculating as I haven’t proven it doesn’t.  It wouldn’t be outrageous to assume another revolver or even a semi-auto would be more like my 357 Magnum than a rifle though.

I'm not claiming that BHN 7-8 is the most accurate alloy in every gun, there are many other factors are at play.  My test results indicate that bullet design and how it fits in the throat/barrel grooves is more important.  Powder choice is critical and tied in importance with fit, as it can and will limit just how tight the groups are (past 15 yards).   Followed by alloy and last of all, diameter (that relationship is important to understand, more coming in a future post).

With the right bullet, like the 358429 Keith loaded at 1.620, any diameter bullet shoots at any pressure without leading (in my Taurus 66 at least).

Though fatter boolits give smaller groups at the 25 yards (my test standard as that's how long the range is).  This is not common knowledge or commonly accepted.  However I’ll post more on other myths soon. 

CLOSING POINTS

After several thousand boolits of testing across alloys, moulds, OAL, crimp, etc... the results are in as far as I'm concerned. At one time I tried to achieve accuracy using pressure & alloy matching.  I accepted this "matching connection w/pressure" concept as valid, it caused me to spend many hours and many boolits trying to figure out what was wrong with my boolits, my gun, or my casting approach. In hindsight it was a good learning experience. But I wish someone just let me know up front...

Those that write about this concept, those that I've read, have extensively tested with rifles ONLY.  I'll try it again if I ever get interested in rifles.  But for my revolver, and probably handguns in general, working up loads based in starting recipes and testing the other variables (including loads) is time much better spent. Alloy/Pressure Matching for Accuracy is fiction in this case.  If you cast for a handgun, it's most likely fiction for you too.

The last word: Pressure isn't an important part of the equation when loading for accuracy.  Knowing what the PSI or CUP is really doesn't matter.  You and I already know that 5 grains of HP-38 is higher pressure than 3 grains of HP-38.  Beyond that, it doesn't matter what it is, as long as it's safe for gun.