Relationship between Velocity and Pressure.

[rusty gunns]

I've been researching a new caliber to reload. As usual, you hit all the resources available. And as usual, the list of bullets and powders becomes mind boggling.

Bullet choice is never a problem. You know what you want to do with the rifle, so you choose the bullet(s) best suited for the job.

Velocity is not too much of a problem. Speed affects flight characteristics, accuracy, penetration, and all the things you want your bullet to do when it hits your intended target.

Pressure. I know the pressures you load to HAVE to remain within the safety margins for your particular rifle.

But here is where my ignorance peeks through. In this particular venue anyway. The entire list is quite long.

Using different powders, you can load the same bullet to the same velocity but the pressure ranges vary widely.

How does pressure affect the accuracy, trajectory, felt recoil ... what you want your bullet to do?

[Terry Murbach]

IT ALL DEPENDS ENTIRELY ON THE PEAK PRESSURE OF THE PROPELLENT YOUR CHOSE AND HOW FAST IT REACHES THAT PEAK PRESSURE. THIS IS ALL SORTED OUT EASILY WITH A HALF DOZEN LOADING DATA SOURCES, AND YOUR TESTING OF THE CHOSEN PROPELLENT STARTING WITH THE BEGINNING LOAD AND WORKING UP SLOWLY FROM THERE LOOKING FOR ALL THE NORMAL HIGHER PRESSURE SIGHS.
I CAN TRUTHFULLY SAY AFTER LOADING FOR 135 CARTRIDGES HERE FOR Y.E.S.T., AND TEN YEARS AT THE AMMO PLANT, IF YOU'VE LOADED ONE YOU'VE LOADED FOR ALL AS THERE IS NO--NO!!!!!!!--DIFFERENCE, IF YOU'VE BEEN PAYING ATTENTION. IF YOU HAVE NOT, ALL HELL CAN BE OUT FOR NOON VERY QUICKLY.

[AJMD429]

Excellent stuff. Next step would be to use the same methodology to evaluate "slow" powders for those pistol cartridges in the rifle barrels and see if the assumption we all make about increased velocities with those powders in long barrels is really true...

[Griff]

What Terry said... if I understood him! If you take notes and pay attention in the load development of one cartridge, the same technique ill stand you in good stead with any other.

Every rule has its exceptions, so... "a higher presure will generally yield a higher velocity, except in those instances where it won't", is really an oversimplification. But in light of the K.I.S.S. rule, velocity velocity can be optimized when burning rate allows peak pressure to be attained at a point where subsequent bullet drag in the barrel is minimized. Which is why I belive most rifles prefer slower ppowders and pistols need a faster one. That last sentence ignores ALL the other factors that affect pressure, but it must be kept foremost in your mind as you develop any given load.

[rusty gunns]

Great stuff, guys, thanks.

And you're right. I have tweaked a few 45 colt and 44-40's to know that your posts are the best way to analyze and develope new loads.

I guess I didn't really phrase my question properly. So I'll squeeze the sides of my head a bit harder and try again.

Pressure tells you how fast a powder burns.


Here is the example I had in mind.
45-70 and a 405 gr lead fp bullet.

One powder @ 28grains yields 1400 fps with a pressure of 17,800.
Another powder @41.5 grains yields the same 1400 fps with a pressure of 15,700.

Now the same bullet is going the same speed with both loads.

What would be the advantage/disadvantage of higher pressure. Isn't the end result the same?

The weight and speed dictates the energy, doesn't it?
(Is this where I'm wrong minded?)

[Buffboy]

Great stuff, guys, thanks.

And you're right. I have tweaked a few 45 colt and 44-40's to know that your posts are the best way to analyze and develope new loads.

I guess I didn't really phrase my question properly. So I'll squeeze the sides of my head a bit harder and try again.

Pressure tells you how fast a powder burns.


Here is the example I had in mind.
45-70 and a 405 gr lead fp bullet.

One powder @ 28grains yields 1400 fps with a pressure of 17,800.
Another powder @41.5 grains yields the same 1400 fps with a pressure of 15,700.

Now the same bullet is going the same speed with both loads.

What would be the advantage/disadvantage of higher pressure. Isn't the end result the same?

The weight and speed dictates the energy, doesn't it?
(Is this where I'm wrong minded?)

Remember the pressure reading is the peak only, so it's all in the dwell time. The length of time the powder stays at a specific pressure. How quickly or slowly it reaches that peak. A slower powder gets higher velocity at a lower pressure because it rises to that pressure slower and stays there longer. A faster powder get there quickly and then drops off rapidly.

[Charles]

Terry of course is spot on. However few of us have access to the equipment and expertise that Terry does. That is why it is so important to have and use good loading manuals published by people with the proper equipment and expertise. When it comes to handloading:

1. All to few people recognize the inherent dangerousness of the activity.
2. Folks who lift loads off the internet from other shooters without checking them against good loading manuals are playing with fire. Lots of folks who post like experts are in fact clueless.
3. When it comes to breaking ground in handloading not covered by the good manuals, don't do it.. UNLESS you truly know what you are doing and very few do.

[rhead]

Pressure creates velocity in only a secondary manner. When pressure is applied to the base of a bullet ,in excess of its resistance to motion,(friction and inertia) the bullet will accelerate. The pressure causes acceleration and the sustained acceleration causes velocity. The pressure wil drop after the powder finishes burning as the moving bullet increased the size of the container. It will continue to accelerate as long as the pressure is great enough to overcome the bullets resistance to acceleration. The pressure of course drops to zero when the base of the bullet clears the barrel. We sometimes get in the habit of equating pressure with velocity but high pressure can be caused by a lack of velocity.(Stuck bullet= very high pressure zero velocity.

[El Chivo]

Last I heard, Velocity wasn't speaking to Pressure because Pressure was too pushy and controlling, and Velocity likes to be free.


As I see it, heavy bullets have more inertia than light bullets of the same caliber, so it takes a little time to get them going.

Think bowling ball vs golf ball. A bowling ball you push down the lane, and a golf ball you whack it down the fairway. If you whack the bowling ball, it moves but not much. And if you push the golf ball, it moves but not much.

If you use a fast powder with a heavy bullet, it's like whacking the bowling ball, you have high power at the moment of peak pressure but not much else to overcome the inertia. And if you use a slow powder with a light bullet, the bullet is gone before it receives all the energy from the burn.

I think barrel length enters into it also. Pistol powders are fast because the barrels are short and any powder still burning after the bullet leaves the muzzle is wasted. Rifle powders are slow because that takes advantage of the longer barrel.

I guess the way to do it is test ad nauseum, and record all your velocities. Then see at what point you run into diminishing returns.