Home Forums Hunting & Shooting Ask The Gunwriters Question about velocity/pressure

In a separate post on barrel cleaning, Mr. Charlie Sisk mentioned the above quote. I'm curious what is happening in this scenario, and how it is expressed in pressure testing. Also, sort of percentage changes in velocity and pressure are testers seeing from the first shot from a clean barrel to stabilization?

Assuming the combustion and the flame front is consistent between shots is consistent and thats a big if, the physical velocity at any given point can be measured, (cutting a barrel at different lengths and chronograph). The bullet velocity would be effected by factors that would change less from one test to the next. The pressure at any point in the barrel is usually a log fitting curve based on data points, either by piezoelectric or extrapolation using differential equations based on flame front velocities. These inputs could have wide swings at the first measurements, between each test, but the extremes would "iron out" after repeated firings.

As I recall there was quite a study done using T/Cs when they were popular and there was an ongoing argument whether longer barrels were more accurate. The single shots were used because different barrel length availability.

Here is something for those who don't use pressure testing equipment: After you get a barrel clean, velocity will usually stabilize by 5 firings. It takes 12-15 for pressure to stabilize.
Yes...velocity can be stable and pressure still having some significant swings.
Charlie

If this is the case than when shooting for groups it would be best to shoot 12-15 fouling rounds because pressure causes velocity swings, and after 12-15 fouling rounds the rifle would settle down, or am I missing something?

What I have experienced is this: lets say you are running a cartridge/powder/primer/bullet at an even 60,000 psi and 3000 fps.
A very good load will have less than 600 psi swings and less than 20 fps extreme spread on velocity. That's with a barrel with that has not been cleaned. Then clean it very good, all the carbon and copper removed. Usually velocity will stabilize in 5 or so shots. But it takes about 12 to 15 for pressure to stabilize, usually 2000 psi or so swings.
But....some loads and barrels may take longer. I have experienced some factory barrels (not match grade) that from a clean barrel to a fouled barrel could change 10,000 psi. Depends on the barrel. And how in particular it was fouled.
And here again I will include this part that pisses everyone off: pressure is described in PSI or CUP, not with adjectives or adverbs.
Don't think that you can measure pressure with eyesight, it wont work. And looking at primers (even if its blown), doesn't absolutely mean anything. Or measuring case heads. And sometimes just because the bolt is hard to lift, because sometimes other things can cause that. Like a front action screw that's too long and hitting the bolt lug.
Now to be fair, I have done very little testing with factory barrels. And that experience has shown me that factory barrels are not as good as match grade barrels. Its that "kinda get what you pay for" thing.
And this: I don't have much experience with cleaning factory barrels. I usually am cleaning match grade barrels with properly cut chambers. That's a whole different ballgame.
Charlie

I think Mr. Sisk was referring to peak pressure. To get the same velocity the average pressure would need to be nearly the same assuming the average drag on/friction between the bullet and the barrel was close.
I’m pretty sure that Mr Sisk is more knowledgable on this subject than me, especially the friction on clean vs fouled barrel.
But acceleration is directly related to pressure pushing minus the drag. With area and mass being part of the equation.

Edit: Oops, I see that Charlie answered before I got my two cents in.

Some loads can have 5000 psi swings and less than 10 fps spreads. When you look at the pressure curves, you will see the area under the curve is the same, just shaped differently.

I truly wish several here on the Fire would purchase some pressure testing equipment. And spend a fair amount of time and money learning how to use it. And I promise, it will be more time and money than you think. One of the most eye opening things I learned first, was that the electric grid in the US has far too swings to be able to use for this type testing. At least in Kentucky and Texas.
And test a lot of different cartridges and powders and bullets. Then, start trying to test primers. That's a real head scratcher.
I used to think I knew a lot about handloading and such until I went down this path.
Charlie

Mr Sisk,
For a given bore diameter do smaller cases yield more consistent pressures or velocities?

Are there specific BG cartridges that you like? I use the 270W a lot and will welcome your comments.

Do consistent pressure or velocity or both predict group sizes?

Do specific powders produce predictable results?

Thank you
R

RinB
I'll need to answer this in pieces:
Usually, smaller cases are better. The PPC line, the BR's and those usually run the most consistent . 221 Fireball runs like that too. The larger cases not so much so. Seems the longer the case, and the more over bored it is, the more difficult to get pressure and velocity you want. But.....and there is always a "but" in this....I have tested the 300 Win mag with WLRM, Magnum, 180 grain Bergers. You can get pressure swings less than 300 psi, and thats about as accurate as the equipment is.
7.62x39 with WLR and Exterminator and match bullets does very well to. Maybe a little better than the PPC cartridges, its hard to tell. But these are in the same family so its to be expected.
On the other end, I built a 257 STW for a customer. That had such big swings I wouldn't let him have it, gave him his money back. For me it was too much liability.

Consistent pressure and velocity is certainly a step in the right direction. But..(damm these buts), it aint a guarantee. But usually if you have both, small changes in seating depth will produce good accuracy.

I bet the 300 Savage would check out as pretty consistent. Short powder column, 30 degree shoulder, modest powder capacity relative to bore size.

About powders...thats a hard one.
Usually, faster powders are used in smaller cases, PPC, BR,etc. Single base extruded powders (IMR in particular) are easy to ignite compared to a double base ball powder. So most of the time, if a cartridge will show good pressure curves, it will with a single base extruded powder. Right up until it doesn't. I have never been able to get any powder in 300 Win mag to work as well as Ramshot Magnum. But a kinda rule of thumb, a faster powder will show a better curve. Most of the time. Usually. Until it doesn't.

And something else that wasn't in your post: BULLETS
Some cockeyed bullets will really throw $h!t in the soup. Hunting bullets wont ever give curves as good as match bullets. Unless you get some junk match bullets, and that happens. But..(another one of these damm buts)...I used my Juenke machine to test bullets when I was pressure testing. If you test and get a batch of say Partitions that were really good, compared to some Berger or Sierra or Hornady that were not, the Partitions would produce a more consistent curve. But usually , the match bullets were better.

Thank you, Charlie. This now makes sense to me. Internal ballistics can be different and muzzle velocity relatively consistent. Friction would be one factor. Maybe, tool marks in throats/barrels and the bullet inconsistencies you mentioned. Alot of mysterious things going on in there. Jordan chasing upswing nodes in barrel movement with seating depth kind of blew my mind. I just wasn't thinking in the super slo-mo of interior ballistics regarding pressure curves while the bullet is still in the barrel. But, that one sentence turned on the light bulb. Thanks.
Andy

Charlie, what equipment are you using to generate pressure curves?

Pressure Trace

Yes.....then there is neck tension.....a huge variable....I learned the hard way.

Yeah, both of us did!

I spent considerable time "experimenting" with Charlie and his Pressure Trace when he was in Texas. Was also lucky enough to spend some time with Western Powders' piezo setup in Miles City--and have visited other pressure labs of bullet companies and one major American ammunition company.

They all provided lots of information, including things not described in manuals.....

Right, makes total sense. I think, the big thing standing in the way of my thinking about this is that MV is one measurable number where pressure is a curve over the time with an infinite amount of numbers along the curve and different shaped curves based on all kinds of factors. And, a curve with a spike might wind up with the same MV at the end of the curve as a differently shaped curve with no spike. I had it in my simple head that pressure is one number like MV. Super interesting.

Using pressure testing equipment (in my case an Oehler 43) can be a very humbling experience for a handloader. The number of variables can almost seem infinite. The number of different types of chambers, barrels, powder, primers, bullets, cases and the means they can be assembled is just a start. And then add in lot to lot variations and the fun compounds.

Logger
You got that right !

Last time I saw John Linebaugh we talked and looked at numbers from Western Powders.

I don’t show them to folks much. Just eye openers.

And that’s in a 500L……..same bullet weight and velocity……..except one was 35k psi…..one was 80k

John loved the study of pressure and velocity. Moving out there and looking forward to reading in his library.

Precision Shooting is another pleasant read.

Thank you for this thread.

One of the tests I performed at the Wester Powders piezo lab dealt with the "accuracy" of the long-time pressure-signs still used by many handloaders. I handloaded some rounds for three different rifle cartridges using such signs as flattened primers, case-head expansion, sticky extraction, and ejector-hole marks on the case heads.

Then I loaded up new cases of the same brand with all three loads and we tested 'em in the Miles City lab. It turned out that one load just about spot-on, another was way under SAAMI pressure, and the third was way over SAAMI pressure.

Ran some other tests in their lab back then, and it turned out that most "pressure signs" except primer flatness don't show up until around 70,000 PSI average maximum pressure--which is 5000 PSI more than SAAMI allows for any cartridge. Primer flatness, however, can occur even with pressures in the 50,000 PSI range, usually due to a little more headspace than SAAMI specs allow.

Any of this pressure peak variance have anything to do with barrel harmonics/vibration? JB, any notice of a more consistent pressure curve with a full-length bedded NULA vs. something with a generous free-float or tip-pressure?

I have tested that and found no correlation .

And there's no reason there should be any....

Thanks to CharlieSisk and the others for their contributions! I don't have much to contribute, but have a couple other questions:

Would this be a factor in the 7mm Rem Mag's lower SAAMI MAP pressure? I've read how wilder pressure swings in that round have been cited as a reason for 62K psi vs 65.

What sort of pressure testing equipment would your recommend these days to someone who wanted to dabble in testing?
Also, when you mention the electric grid swings, are you saying the variables in grid AC power introduced inconsistencies into your results?
This is good stuff, thank you for posting.

Interesting thread. Can you (John or Charlie) tell if you have experienced a pressure change with faster or slower twist rates?
It was an argument in another thread here years ago.

Jeez - it's a wonder the average joe reloader can get things to work as well as they do.

PGoat
I have not experienced these swings in 7mm RemMag. But...(dam thing came up again )...the testing I have done in this caliber is with the rifles I have built. Properly cut chambers, good barrels, etc. With good components it seems to work as well as any other.
Except for one time....I had a batch of new Norma or Lapua brass in 7mm Rem Mag. Built a rifle, it shot like $h!#!. Velocity, accuracy, pressure, all over the place. Then, I learned about annealing, and all those problems went away.

I have not experienced any connection between twist rates. That doesn't mean its not there, but I cant find it.

Twist has a very minimal to no effect on pressure

Most electronic test equipment has stabilized distribution transformers, and in most cases are DC. NMR,IR,Mass Spect., Xray diffraction. These usually cost more than the measuring device. It depends on how many zeros to the right you want to go. The pressure equipment uses an algorithm that is built into the chip, and gives the time/length pressure curve seen on the observation device. If measurement interval is made smaller this curve will have peaks in it not "smoothed" out by the fitting log formula.

Understatement--- good thing those random errors cancel out the systematic ones.

I have a strain pressure measuring system. It's a Faber unit before Ristow offered his Pressure Trace (I also have an unused PT). The Faber captures Peak Pressures (actually it produces a number that he termed "microstrain" that I have to convert to psi via a closed vessel formula. I primarily used it to work up service rifle loads for the 90gr JLK (to keep from blowing ourselves up in uncharted territory).

Funny I just decided I needed to get a PTII and found they're discontinued. I have an email in with Schaefer with fingers crossed.

I'm not sure that is a question that strain measurement can answer definitively. Mounting a gage on a barrel makes it a unique system that provides strain measurements. You then have to calculate the pressure given a multitude of factors such as barrel dimension, brass hardness etc etc. Mount a gage on a different barrel and you've created a different system subject to it's own calibration etc. Calibration between barrels (systems) is a challenge unless you're prepared to use a high pressure oil system as Harold Vaughn did or have access to expensive high pressure measurement equipment. I had a reference lot of ammunition that I'd fire before each session to ensure all was as expected. As imperfect as that was, it was as simple and practical as I was willing to go. Ristow felt that his computer algorithm did a good enough job with calibration. I was skeptical.

From a purely engineering perspective, the energy required to increase spin on a bullet (for us it was going from 1/8 or 1/7 to 1/6.5 was neglible. This was borne out in our firing tests. Same load in same chamber design shot to pretty much the same velocity whether it was in an 8, 7 or 6.5 twist.

Mr. Sisk, I'm curious how you're calibrating your PT? Are you going with the software calibration?

Most experienced handloaders eventually learn that there are standardized or common velocity thresholds where pressure safety and accuracy can be expected as a reasonable outcome. longer case life will support those velocity ranges per cartridge and the loading manuals have been teaching us that for decades if we listen.

Once a hunter learns that chasing that extra 100fps doesn't mean much in the field and could be the reason that the extra level of accuracy you are wanting is absent, things get a lot easier and a lot safer for the rifleman and his components.

If you want magnum velocity, great, but get it from a magnum case. Life can be simple if you're smarter about it.

Well and succinctly said, AGW.

Thanks Charlie and Chris. My thoughts were same, jwp. The amount of energy to twist it IS FAR less than needed to launch it. But inanet warriors thought otherwise.
And, Chris, yes it would be very hard to test for being in different barrels.
Thanks all.

A comment on twist affecting pressure:

It has been demonstrated many times that peak pressure mostly depends on when the bullet is being engraved by the rifling--more-or-less shortly after leaving the case mouth, depending on the length of the chamber throat.

Which is why Weatherby started using considerable "freebore" (longer than average chamber throats), which flattens the pressure curve. But longer "freebore"doesn't necessarily result in more velocity for the case's powder-capacity/bore ratio.

Spent two summers playing with a M43 a long time ago. We didn’t have a clue what we were doing when we started out—even computers were new to us! The learning curve was challenging…

I still open up my binder with the printouts of the pressure curves to review them once in a while.

My 30-06AI still has a strain gauge glued to the underside of the chamber……..😄

Sorry for being a bit slow…
But I’m missing the tie to twist above. Can you elaborate?

I have yet to see (or hear) of a test where peak pressure wasn't reached within a short distance after the bullet left the case and entered the rifling. Say 2" on average.

Beyond that the pressure starts dropping, due to the greater volume inside the bore behind the bullet. Have yet to see/hear of any test that showed a significantly higher pressure peak due to a faster twist. The biggie is the pressure required for the bullet to enter the rifling.

The pressure peak is caused by the acceleration of the mass, ie overcoming the inertial of the bullet, while any gain in twist only occurs what ever the twist rate is ie must travel 7 in or 8in etc. to complete 1 full rotation. The effect is marginal on the pressure curve.

Once cleaned my Sako 85 300 WSM. It took 10 rounds for the accuracy to return. That surprised me.

Rotational acceleration can be calculated but like MD and others wrote, it’s insignificant compared to other factors such as linear acceleration, deformation of the jacket due to rifling/friction.

The main thing I don’t get for sure is the secondary explosion which is uncommon and I have not read a credible explanation or maybe I just forgot. But here’s my guess: the friction is great enough to reverse the acceleration and pressure builds/spikes enough to become an explosion. I suspect this happens mainly with a powder charge that is in away from the primer and the powder doesn’t sufficiently ignite until a short time after initial ignition. From what I’ve read, it seems to occur mainly with partially full case of slow burning powder if I recall correctly.
My apologies to others who explained this phenomena better than this.

Thank you for indulging a kid from the short bus. I do get your point now. Could I paraphrase that as peak pressures tend to happen when the bullet hits the rifling (before twist can become a factor)? Makes sense to me, but I've not seen pressure curves expressed as Distance/Pressure. I've only seen Time/Pressure. I think Muzzle Velocity might be a better indicator for pressure caused by twist because it would be a product of pressure AUC. Peak Pressure as you note probably doesn't touch differences in rifling twist since it likely happens at the throat before the twist exerts it's effect.

The guy you would have wanted to discuss this was Jim Ristow, owner of Recreational Software and designer of the Pressure Trace. He was pretty proud that his device documented several instances of Secondary Detonations. He'd talk your ear off about it if you gave him the chance. I wasn't too interested in the phenomenon back then, so I can't tell you what his theory was. Like in so many other things, I wish I had paid more attention.

Exactly. I’ve modelled the physics and gone through the math on this forum before. The effect of twist rate on bullet exit speed is minimal.

Rabbit hole just got infinitely deeper....
One thing I need explaining pertains to my load development of my 22 Hornets. H110 and LilGun have almost equal burn rates (ranked 60 and 61 in my chart) Always thought pressure and velocity go hand in hand, however LilGun produces much lower pressures than H110 at similar velocities. not just a trivial amount either. What am I missing?

Meaningless for myself. I have never seen a measured difference that couldn't be accounted for in different barrels, chambers, cases, etc. I have heard people spin tales of quicker twists of being soooo much slower than a couple inches of twist rate and for myself I don't pay attention and just add RPM's..

It's integrated pressure and velocity that go together.

Modeling of explosive wave fronts, detonation and deflagration (Tannerite) is an on going science. Using the pressure front as a driving force is common among firearms, combustion engines, some types of welding. By using the "averaging" method of log curve smoothing the peak and the bullet velocity can be calculated at any length in the barrel and is pretty flat after the first 8-12 inches.
Also the bullet must go the length of the twist rate to complete one revolution regardless of the spin rate. EX. 12in twist, at 100 yd. will make 300 revolutions and obvious both are a function of time of flight.

In any case, all of the models that exist today are linearized (vs. non-linearized) meaning in this case, the time constants are almost completely ignored to uncomplicated the modeling. Non-linear dynamics using difference equations and changing the iteration to smaller intervals will open up new horizons allowing analysis on very small time scales, and three dimensional computer analysis. Calculus is high school math

As mathman says the pressure works over a period of time to accelerate the bullet. A short duration/peak of pressure may not do as much work as a lower peak but longer duration.

below is an abstract taken from a NASA paper describing "vorticity" in flame fronts, referred to in linear analysis. (here and other gun mags). if your interested google non-linear detonation analysis

The interaction of an oblique, overdriven detonation wave with a vorticity disturbance is investigated by a direct two-dimensional numerical simulation using a multi-domain, finite- difference solution of the compressible Euler equations. The results are compared to those of linear theory, which predict that the effect of exothermicity on the interaction is relatively small except possibly near a critical angle where linear theory no longer holds. It is found that the steady-state computational results whenever obtained in this study agree with the results of linear theory. However, for cases with incident angle near the critical angle, moderate disturbance amplitudes, and/or sudden transient encounter with a disturbance, the effects of exothermicity is more pronounced than predicted by linear theory. Finally, it is found that linear theory correctly determines the critical angle.

one study that found good correlation

Explanation ignores the ever increasing volume of the gas mixture, and violates the General gas law. PV/T=P'V'/T'

PRESSURE QUESTION? a cartridge case that is 95 -100 % filled compared to a 70 % - 85 % 0r less can the cartridge with less powder sometimes have more pressure problems using the same type powder ?

Not if you consider the increasing number of gas molecules that occupy the volume. PV/(nT) = P’V’/(n’T’)

Less fill means more variation in powder position and density, which would result in more variation in the pressure curve.

Interesting how we wallow in the ballistic minutia. Much more fun to shoot & hunt.

i wonder if anyone ever posted those kinds of results ?

The Mole ratio stays the same

Yes that's true, but its the interest of the topics being discussed that has developed the rule of thumb reloading methods, gun manufacturing etc.. that has allowed a larger participation in the fun side.

Explanations for the non technical reader often require some simplifications.

I haven’t looked deeply into the specific “burn” rates, but the deflagration process involves varying time dependence of the phase change of molecules from solid to gas.

So I ran an unintentional experiment this morning with my 338 WM. Wanting to test some velocity starting points with new brass and bullets I decided to make 5 blow off rounds to shoot through a squeaky clean bore to begin with. For those five cases I used SIG 7MM RM fired brass, decapped them and ran them through a FL sizing die only. I did not run them over a mandrel to size the necks (neck tension was skookum) nor did I anneal the cases as I normally do before sizing. I did nothing else to the case. I added a primer, 71gr of IMR 4451 and a 225gr SST seated to the cannelure.

Here are the results. I wouldn’t call this velocity finding stability.

Have you ever noticed how handloaders, me included, chase the last foot second from our loads that don't compromise accuracy?

Have you ever noticed that factory loads commonly don't reach factory specs leaving 1-200fps gaps between factory and your handload?

Have you noticed when you hunt with fcatory ammo than animals fall over with well placed shots?

We never learn do we.............

Rookie stuff..

First question I would ask is "how are you quantifying pressure?"

Assuming peak pressures are the same, some powders are apparently showing a slower tail off in pressure basically increasing the "push" down the length of the barrel (versus just kicking it in the ass out the case). In other terms some powders increase the "Area Under the Curve" or AUC in the Time/pressure curve. That's kinda been the holy grail for powder manufacturers with people experimenting with duplex powder loads (layered charges of different burn rate powders) and other strategies. Some manufacturers have apparently cracked the code on that (or so says "Johnny's Reloading Bench YouTube Channel").

That phenomenon is well known by older highpower shooters. The M1 Garand 30-06 loadings were limited on powder selection because of the requirements of the gas system. Slower burning powders that we today accept as providing optimal loading density (like 4350) can't be used in Garands without risking damaging the gas system. Powders in the acceptible burn range like 4064 and 4895 didn't fill the case and left alot of opportunity for variation in positioning in the powder charge. An old highpower trick was to tip the barrel upward to settle the powder against the primer.
The introduction of the 308/7.62 NATO chambering (and the M14) brought an immediate increase in scores. 7.62 chambered Garands saw similar improvements. It wasn't that the 308 chambering was more accurate...but rather the powder constraints placed on the Garand handicapped the accuracy.

I've not experienced pressure increases that can be attributed to twist rate, alone. Conclusion is based primarily on MV of same loads shot thru different twist rate barrels. Fastest twist tested was 1:1 (looked "threaded" more than "rifled").

On the "secondary explosion", I've never seen that documented via the piezo pressure measurement technique. If anyone has one, please post it as it would be fascinating to review.

I am quantifying pressure by the Hodgdon 2024 Reloading Manual. I seriously they would print bogus data of that nature.
40 Gr SPR SP 13 GR LilGun 2826 FPS 28,400 CUP
40 Gr SPR SP 11.2 GR H110 2795 FPS 41,800 CUP
What I don't have a clue how you get 31 FPS difference with 13,400 CUP difference, with the slightly faster charge getting that much lower CUP. LilGun is magical beyond my expertise. Thats what I was hoping one of the many smarter fellows than I could explain why and how.

Those numbers don't tell you the entirety of the pressure curves.

Nor are they all that precise compared to piezo pressure measurements.

All "science" measurement devices have limitations. Understanding these and building models, math or otherwise, to fill in the gaps is part of the methodology. There are many "Black Swan" events in a single cartridge firing.

Measurement uncertainty is ever-present, and the precision limitation of the device is only part of it. For those interested, the GUM (Google it) will tell you all you want to know and more. Fair warning- it takes some effort to get undergrad physics students to understand the nuances, as it’s not a light read.

Another good tool box is Matlabs Neural Network toolbox

Ristow's pressure graphs are available on the wayback machine. He used Strain though, not piezo. I'll see if I can link some stuff in here.
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