Home Forums Hunting & Shooting Ask The Gunwriters Pressure increase, can you predict it?

I was looking at some data for several cartridges. I looked at the start loads, and max loads and took the pressure and divided it by the velocity to get a psi per fps number.

For instance, a 30-06, 150gr start load at XXXX velocity and XXXX pressure was 17psi per fps. At the top end load it was 19psi per fps. And for other 30-06 based cartridges that seemed to be about the norm. 17-18psi per fps of velocity at the low end, and 19-20psi per fps at the top end. I just randomly chose loads in the 30-06, .280, and .270...so there may be some differences.

My question is, knowing this information, can one reliably predict pressure increase by velocity? Does the increase remain constant, or does there come a point on the low end where it would decrease dramaticly, or increase exponentially at the top end?

I hope I asked that right.

I'll be watching for the answers by some of the Gurus on board which I'm not. The chosen cartridges are some of the most efficient out there in comparison to the larger mags- which score poorly.
Excellent question.
Roy Weatherby also played with this idea and the only way he could increase velocity after a certain point was to freebore the chamber so the bullet jumps to the lands increasing the muzzle speed.

I had one load with a ball powder that actually slowed down when it got over max, and it pierced the primer, which told me it was too hot before it even got near book max. That was early shooting with my new chrono to see where some of my loads stood. I have since learned that velocity will go up on a more or less predictable level, then a sharp turn up, or down, usually signals that pressure may be too high. Some types of powders are different in how they react also. Double base and ball powder will show fine and then one more step knocks the primer out, whereas single base powders tend to be alot more linear, or they give some sign before things get bad. Like sooty primers or a shiny spot on the case head where the ejector plunger is located. To really answer your question I think your observations are right on.

Once you have reached max safe working pressure and velocity (with the proper burn rate powder), as you add more powder pressures can rise exponentialy at the top end.. Sometimes dramaticaly....

+10 you got it for smokeless powder.

The short answer is no.

It's not as predictable as it might seem. As some of the other guys have pointed out, it might SEEM to be a simple step-up thing - until it suddenly isn't. but by then, you've already passed the safe zone.

Or, as denton would say, the charge to pressure curve is not linear.

BTW, the reason you see a difference in your psi/grain list is due to bore diameter. The smaller the cross sectional area of a bullet (smaller bore) the more pressure it takes to achieve the same "push". PSI does stand for pounds per square inch, so the fewer the square inches, the more pounds it takes to get the same result.

You can predict that, or at least the velocity change. It's another 1/4 rule. When case capacity and bullet weight remain the same, velocity will vary at one fourth the percentage change in bullet base area. A bullet 8% larger in base area will be about 2% faster at the same pressure.

I posted a while back in a thread that I didn't think pressure increase was linear. Another poster said it was. I contend that at somepoint the amount of increase in psi per fps rises sharply at a couple of points along the pressure line. Which why we not only have max loads but minimum loads too. There's that "window" where pressure can be reliably predicted to increase along a certain slope, but outside that window that slope may increase or decrease dramaticly. IOW, if I'm loading for my .280 with a given set of components, within that window the slope might go from 18psi per fps to 20psi per fps, but then somewhere at the top end that slope is going to drasticly increase to like 25 or 30psi per fps in just a few grains of powder and/or fps increase.

Is that a fair statement of what those that have answered are saying?

I think that's fair. It's a bit like saying roads are straight. They are in places, but not forever. The center portion of a charge/velocity graph may be fairly linear (and that's the portion that's generally posted in the load manuals). But outside of that linear section, the "road" may take a sharp bend.

That's what I thought, and yours is a good description. I had suggested that, and another poster said I was wrong, and honestly I can't recall the poster...and it doesn't matter. I would also imagine that this has much to do with how SAAMI sets is's "Maximum Avg. Safe Pressures" because that would be at or just below the level where the road might start taking sharp turns for a given cartridge.

When nearing the top end loads, a point will be reached where a small increase in powder gets a rather high velocity increase, this is near the point of dimenishing returns. More powder will start to show very high pressure with minimal to no velocity gains...

I've looked at published data, too, and I can't see any pattern I'd care to bet my eyesight on.

Generally, pressure rises about 3 times as fast as velocity. That is, a 5% increase in fps will come with a pressure increase on the order of 15%. This varies with the type of powder. Perhaps due to experimental error, it even varies among cartridges using the same powder, even when case capacity and bullet weight are scaled to the bore cross-sectional area.

With CUP style measurements, the ratio is smaller, most likely because CUP seems to increasingly underestimate pressure as the true pressure rises. The old Powley Computer (written for CUP style data) assumes a 2:1 ratio of pressure to fps, but I've found that 2.5:1 is closer to modern CUP test data.

In short, you can't reliably chronograph pressure.

Get yourself a copy of Quickload. You can model pressure and velocity with incramental load charge variations.

Rocky has it right with the example of being a section of straight road without knowing what is around the bend. I go even further with saying that the straight road example applies only to your lot of powder, in your gun, with your primers, at close to your charge level, at the same seating depth, ... and even the color of your shirt. Change anything and you might be around the bend, or you might be on a different road entirely.

We can discuss all these pressure measurements like angels dancing on the head of a pin, but in many cases we are trying to apply tenth or half percent precision to a process already having five or ten percent uncertainty. If we knew and understood the whole process, then all loading manuals and even QuickLoad could agree. SAAMI recognized this long ago when they published conservative maximum pressure levels which are routinely exceeded by individuals who have seen no problems --- yet.

Pressure and velocity are related. You can't get velocity without pressure. Increasing one usually implies an increase in the other. That's as close as I dare tie it down until I do some actual measurements on a specific load in a specific gun, and then I'll place bets only on that load in that gun.

KenO

Ken, good points, and thanks.

Let me ask you this based on the above you wrote. You say SAAMI's standard are "conservative"? Why? To that end, if a loader could know he was operating at , not over but at SAAMI maximum average safe pressure in a given gun, that he'd be "running the ragged edge", or "doing the speed limit?"

I have pressure tested data for several powders using a 30-06, 180 and 200 grains pills. Max psi was 67,000 I think. I was interested in pressure/grain of powder and graphed accordingly.

The graphs look fairly linear for the powders tested (mostly toward the slower end). I was seeing ~ +2500 psi/grain of powder and ~ 40 ft/sec/grain using the 06, 180's and 200's.

From what I saw, it would be safe to assume linearity if you keep your head on straight with respect to margin of safety. If your goal is to push the envelope - say 65,000 psi for an 06, I'd strongly advise against assuming linearity.

Conceptually, I think pressure is what drives the non-linearity. Pressure makes things weird as compared to standard pressure and temperature.

Bottom line: We have enough pressure tested data to not have to experiment much. If you want to add 1 more grain to an 06 charge, thats likely really safe. If you want to guess your 257 Wea "isn't shooting as fast as it should" and want to add a grain - Good Luck. You'll need luck and divine intervention to repair your wrong guessimate.

Good question.

I call this performing surgery with a chainsaw.

QuickLOAD can tell you which powders are your best ("optimum") choices for your combined criteria. My criteria, for example, are
(a) maximum pressure not to exceed 50,000 lb/sq in. (for safety)
� and �
(b) 90�100% case fill (for consistency).

If you use one of the optimum powders, you won't be able to drop enough of it into the case to get dangerously high pressures. That happens when the powder that you're using is too fast for a caseful of it to produce safe maximum pressures.

If you use a powder that's too slow, you can't drop enough of it into the case to get pressures as high as you want.

As illustrations, consider the extremes:
� Bullseye is 'way too fast for a caseful to be safe in a .30-06,
� and �
� 50BMG is too slow for a caseful to produce the pressures that a .30-06 can handle or the velocities that a .30-06 can safely produce.

The optimum powder for any case, bullet, and seating depth (net powder capacity) is a matter of quickness (erroneously called "burning rate") and bulk density � not a matter of personal preference or availability, or whether it's optimum for another cartridge. The optimum powder for any round is one of the available powders that
(a) just fills the net powder space in the case
� and �
(b) produces no more than the desired maximum pressure.

Any other powder is either too fast (dangerous) or too slow (weak) to give you the best combination of maximum velocity and safe maximum pressure.

Ken,

That's as fine as I've ever seen that put!

If the loader knows that he is operating at the SAAMI maximum average pressure, then by definition he is "doing the speed limit". The question is, just how does he know exactly what pressure he is running in his gun?

What the "same" load did in another barrel doesn't transfer.

KenO

No, no tricks...my question was meant to define the SAAMI max. avg safe pressure, notthing else. Some loaders I know think SAAMI max is the pedal to the floor...I veiw it as the speed limit. "Knowing" is a whole different question. And you're dead on, without pressure testing equipment and the knowledge to use it...it's all just a SWAG made by applying data and experience.