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

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.