Pressure signs vs velocity

Velocity changes are an indicator of pressure changes. But they are a poor indicator of actual peak pressure.

 

Going back to the original post, the question is, when I replicate the components in the book, does muzzle velocity indicate pressure? The answer is, it does, and very well.

Further, peak pressure is very highly correlated with the area under the pressure curve. MV is well correlated with both total pressure and peak pressure.

If you have the same length barrel as the book, and are using the same components, and are getting book MV, you are also getting book pressure. I have made many measurements of pressure and velocity over the years, and this holds up very well.

 

Average pressure correlates best to velocity (in a relative way). Not so with peak pressure. And peak pressure doesn’t necessarily correlate well to average pressure (area under the curve).

 

I havent been reloading near as long a lot of guys here but I dont know if I have ever used all of the listed components let alone had the same firearm as used in the test.

 

But it does, at least if you are sticking to the question posed in the OP.

The length of your barrel is fixed. The integral of pressure over distance is the work done to accelerate the bullet.

You get more area under the curve by increasing the peak.

Total area under the curve is very highly correlated with peak pressure.

 

It’s been 20 years since Calculus III, but I thinks it the double integral of the pressure curve to find the area under the curve. Maybe?

 

This may or may not be correct depending upon a lot of variables. Burn rate of powder, duration of burn, etc...

I also seem to remember that a higher peak doesn't always mean more area under the curve.

chris

 

I think we are looking at the situation differently. You've seen good correlation between peak pressure, average pressure, and velocity from a specific set of components in a specific firearm. I believe that.

The OP is using a different firearm with a different barrel than Alliant used, and has no pressure data from his firearm. I don't have enough experience to suggest that matching velocity results from his chronograph to Alliant's published velocity is an accurate way to verify peak pressures reached in his firearm. Perhaps your experience is sufficient to do so.

 

I think that might be the worst thing about chronographs and published velocities with load data, people trying to achieve some written number.

How about not starting at “max”.

If the book says 4.7 @ 1140 max, how about you start at the beginning and see if the starting load of 3.8 gets you 950 fps, or whatever the starting load is. If your data doesn’t equal the book at the beginning, it would be unreasonable to assume you reach “max” at the book’s velocity.

 

bullseye!

murf

 

F=ma.

The bullet cannot refuse to accelerate according to the net force applied to it. Think propulsion force minus friction and engraving force and energy diverted to angular momentum of the bullet. Further, the bullet cannot accelerate more than the applied forces dictate. That would violate Conservation of Energy.

The general shape of the pressure curve is fixed. There are no propellants that give a rectangular or a U shaped pressure vs. time or pressure vs. distance curve.

Given those constraints, if you are using the same type of powder and the same bullet mass as the book, muzzle velocity is highly determined by peak pressure.

If you take the book velocity and adjust it to your barrel length, when you reach that velocity you are very close to the peak pressure that the test loads in the book generated. Seating depth, leade, brass thickness, and chamber tightness all influence pressure, it's true. But more or less pressure produces more or less muzzle velocity.

SAAMI does not just specify pressure. SAAMI also specifies velocity.

Sierra's ballsticians have publicly stated that you can increase or decrease charge to get book MV (adjusting for barrel length, of course). John Barsness has been saying that for years. Federal Cartridge adjusts charge to get specified MV. And, yes, my own measurements on several different rifles support the same conclusion.

 

Good luck to those that try. However, that’s not what I use a chrono for. In regard to this thread, if I duplicate what the published load data has, and am under the velocity published, is there some confidence I am also under the published pressure and am operating in a safe range.

I don’t have the capability to measure pressure. I can only measure velocity. With pistol, I’ve observed some high pressure signs (blown case web) and knew something was wrong, but wasn’t willing to test those loads with a chrono to see what the velocity was. The collective advice from this forum is that you’re over pressure limits by the time you actually see pressure signs. I really only want to stay safe!

 

https://saami.wpengine.com/wp-content/uploads/2019/02/ANSI-SAAMI-Z299.3-CFP-and-R-Approved-2015-12-14-Posting-Copy.pdf


“Due to the fact that sporting firearms for general distribution are typically manufactured to dimensional tolerances greater than those specified for test barrels, there should be no expectation that these velocities can be duplicated from any test utilizing firearms.”

,Bullet type, burn rate and barrel length, throat, seating depth (jammed/jump).

Estimating pressure from a chronograph reading is indirect at best. Nothing other than direct measurement will give you a certain peak value.

The average pressure behind the bullet during its time in the barrel is proportional to the square of the bullet velocity.

Average pressure is a combination of peak pressure and the much lower muzzle pressure.

Muzzle pressure varies more proportional to powder charge than the more exponential peak pressure. Velocity can reflect a larger change in peak pressure than muzzle pressure, how much more depends on the the curve and what part of it you are in.

 

But other factors can produce signs of high pressure that are not really high pressure. For example, excessive headspace will produce cratered/flattened/blown out primers.

 

keep an eye on your ejected brass. if factory ammo ejects brass six feet out and your reloads are ejecting brass 12 feet out, you may be over max pressure.

luck,

murf

 

SAAMI lists 'nominal' velocities. Example, 357 max lists speeds for 125 grain bullets from 1220 fps to 1425 fps.

Also, selecting the right powder can significantly change speed. Example, SAAMI lists for the 38 Super 125 grain bullet a nominal speed of 1230 fps. But using N105 or A#7 can push that bullet to 1450 fps and still remain within normal SAAMI pressure limits. https://www.shootingtimes.com/editorial/super-powders-for-the-38-super/99160

 

Ejection distance is a function of slide velocity which is a function of slide weight and various springs, not chamber pressure.

 

then maybe i misunderstood newton's third law of motion. chamber pressure is the force that is acting on the slide. more pressure means more force applied to the slide which means more slide velocity. more slide velocity means a more forceful case ejection. a more forceful case ejection means the case lands farther from the gun.

you can run these steps backward to understand that cases farther from the gun means more chamber pressure.

murf

 

True. But it does depend on the powder. Some are pretty linear.

 

But consider data such as this:

For the 223, Hornady lists a max load of 24.9gr. of Varget behind the 60gr. Vmax bullet, using Win cases and primer. With that load they report a MV of ~3000fps in a 26" barrel. Hodgdon lists a max Varget load of 27gr. with the Hornady Vmax, with the same brass, and primer. Hodgdon reports a MV of 3159fps with their load.

Using your line of reasoning, there seems to be only two ways to reconcile this:

1) Hornady's data is for multiple bullets and the actual Vmax data is 2.1gr below max.

2) Hornady's data is limited to substantially less pressure than the Hodgdon data.

Are we to assume only one source is correct? And if so, which one?

 

Pressure and force are not the same. Force is pressure times area in this case. More pressure on the slide (actually force) does mean more velocity for the same recoil spring. Velocity of the slide is a function of the mass of the slide and the stiffness of the recoil spring. Using a stiffer recoil spring reduces the case ejection. Looking at how far the case is ejected is useless for pressure indications.

From this link on Enos, the post written by IDescribe, one of the signs of over pressure, maybe the best in my opinion, are that the velocity increase becomes non-linear with increases in charge.

https://forums.brianenos.com/topic/221357-how-do-you-use-a-chronograph-to-work-up-a-load/