Noob question: Different velocities, same chamber pressure.

[breadbasketbomb]

I was reading some loads from Accurate website. Mainly for standard pressure .45-70 loads, being described is 28,000 psi or less. They labeled many powders, but 5744 and 2015 caught my eye. Both their 405 grain bullet loads, same design, had different max load velocities. 5744 was firing out 1600 fps, while 2015 firing at 1800 fps. Yet they were described as having the same max chamber pressure. Am I missing something?

I got the load data from this sight.

www.accuratepowder.com/load-data/

[jimbob86]

Pressure and velocity produced are related, but not directly ..... the different powders may produce the same maximum pressure in the chamber, but the curve ( graph of pressure produced over time) will be different ..... and the listed velocity in their data is just that: their data. Change anything- temp, primer brand, barrel length, even a different barrel of the same length and velocity will change.

[74A95]

You're not missing anything. Different powders produce different velocities even when producing the same peak chamber pressure with the same bullet.

[briandg]

There is an impossibly complex set of important characteristics involved in every load combination. Most powders have their own individual burn character, that decides pressure and velocity. Every case shape gives a different burn profile. Changing caliber and bullet weight changes how pressure builds up in the case and is important factor in eventual velocity. Even the shape and profile of the bullet matters, as this can affect how much resistance the bullet has to overcome, and it affects the way the pressure builds and the powder burns.

If all of those individual things increase or decreasethe potential pressure in a load. We have computer programs now that do an amazing job of predicting results.

[RC20]

I agree on the whole of it.


I would call it an amazingly complex set of variable, but not impossible.

It does reduce down to some simple end results.

A primer makes the power burn, the bullet goes down the barrel and we can see what combination of the factors we affect make things better or worse.

So, its really pretty simple!

Note: Realoder 17 is an example. Its a different powder in that it reaches its peak, but then burns steadily at that peak with a longer duration than other powders.

You can get higher velocities out of it despite the fact that its peak is no higher than anyone elses.

Burn rates are a key and look at where they are in the charts (its a clue, not a given)

[briandg]

I call it impossible because it literally is impossible to take all of the factors involved and determine how each one will interact with the others.

You can add five flavors of syrup to a slow cone, and until it's actually poured and tasted, you can't know how disgusting a coconut blueberry bubblegum hot pepper and watermelon mix can be.

If it was predictable, ballistics labs would spend so much time shooting and measuring.

It took many years before Barnes started cutting bands in their copper bullets.

[briandg]

By the way, mister bomb, this is why you should never use anything but data drawn from official sources given by the various companies. Data taken from unofficial sources has not been properly tested.

[Stats Shooter]

Different powders giving same velocities with different pressure, and powders giving same pressure and different velocities is one reason why there are so many powder and bullet combinations.


The "same velocity and different pressures" issue is one of the biggest reasons GI's in Vietnam had issues with the M16. Armalite and stoner used a different powder when developing the gun/load. According to my book titled "White Feather" (about Hathcock but went into the early issues with the M16 in some detail) Stoner and Armalite used an extruded powder with a longer, but lower, pressure curve. Then when colt took on the contract, they used a ball powder with much higher peak pressure and cases got stuck.... often ripping the casehead clean off. Colt worked for years under the assumption that it was an issue with the rifle, but it mostly turned out to be a powder problem.

[T. O'Heir]

"...the same max chamber pressure..." The Max pressure for a particular cartridge is set by SAAMI(Small Arms Ammunition Manufacturers Institute), Stateside. It'll be the same regardless of the powder used.
There's two ways of measuring pressure. CUP(Copper Unit of Pressure) and PSI(Pounds per Square Inch). The two are totally different methods of measurement and there is no mathematical formula for converting one to the other. Nor are they interchangeable. All this will be covered in your manual.
That 28,000 PSI or less applies to BP using Trap Door rifles too. Modern rifles like Ruger No. 1's (that can handle .45-70 ammo loaded to near .458 Win pressures) and lever actions Max pressures are higher.
"...syrup to a slow cone..." That a 'snow' cone? snicker.