Can a bullet slow down before leaving barrel?

[MEHavey]

The joke's on you.

I'm assuming you are agreeing w/ me . . . that bullet slow-down while still within the barrel . . . is not some rumor-pseudo-hill-billy-physics theory.
. . . and that we both make use of it -- and for the same reason.



Interesting that we were both preparing the same reply, using similar data, and the same tools . . . at the same time.

 

[Bayourambler]

What is the purpose of this pursuit? Trying to achieve some theoretical goal? And the benefit of this is what?

The benefit of all of this is to get my mind off of the daily stress of life and work related problems. I dont do well with an idle mind and reloading gives me a hobby that occupies my mind and gives me stress relief. Also , there is just something about making a load that works out good and hits what we are aiming for way the heck over there!! You know what I'm talking about, that's why you are here!

 

[Varminterror]

The instant the powder is burnt that bullet is only going to slow down.

Very disappointing statement to hear from an engineering student (said as a career engineer myself. Study internal ballistics for powder gasification, combustion, and bore pressure curves for about 20min some day and you will see the fallacy in your statement.

Powder is gasified early in the bore, then the gases are combusted as the bullet travels.

The combustion reaction converts a relatively high density solid powder into a high pressure gaseous mixture. The “instant the powder is burnt,” the bore behind the bullet is full of high pressure gases - as you’d realize as an engineering student, pressure is a force applied over an area, and in our particular physical system, the bullet will remain accelerating (albeit variable acceleration) any time the force behind the bullet exceeds the friction of the bearing surface on the bore wall plus the relatively infinitesimally small inertia of the air in front of the bullet. So no, the moment the powder is burnt, the bullet does not start slowing down. The powder is burnt, it remains a high pressure system on one side, unbalanced, such a net outward force remains until the expansion is satisfied. PV= znRT at that point....

ETA: so your free body scribbling isn’t necessarily incorrect, but your basic understanding of when the propelling force of the system declines is the problem.

 

[Varminterror]

So once a barrel heats up, the pressure may be just tiny bit more than the last shot

If we’re counting angels on the head of a pin:

This may not be true in all cases. Theory might suggest the gas pressure is slightly higher at a slightly higher gross temperature (less heat sink as the barrel heats), however, metal expands as it heats, and such, the bore diameter also expands slightly, which will reduce the bearing surface drag on the bullet and increase the cross-sectional bore area . Considering the relative magnitudes of influence of the barrel heat on these two individual rabbit trails, the metallurgic expansion will be the dominating factor over the lesser pressure increase by higher gross temperature. Recall, the combustion is happening at VERY high temps, so a barrel temp increasing by 50-100 degrees really isn’t a huge shift in the gross maximal temp, but it IS enough to create a dynamic in our bore diameter via metal expansion, small numbers on small numbers, as it were.

 

[crestoncowboy]

I don't know about the OP,
But ive personally dealt with this on two occasions, where the barrel length wasn't optimal for the ammunition. Once to find the best powder in a 22 inch barrelled 44 mag. And once to find the best (faster) powder for a short barrelled 300 magnum.

As far as the Condensed version of the events taking place by an engineer, I think we take shortcuts to avoid taking up to much time and farther complicating things.
One of my long time friends has his phd in chemistry. I can think of many times he gave simple condensed (while not entirely accurate) explanations. Once to a moonshiner he said "the yeast eats the sugar and s**** out the alcohol. And another time when ethanol was starting being used in gasoline he explained to an older small engine mechanic that the "ethanol would turn to syrup and gum up carbs". On the way home I got to hear the 45 minute version with scientific terms and formulations. I prefer the "syrup" explanation. Both were fine explanations to explain what needed to be resolved. He could have written a 20 hour thesis on either topic with different variations and all the chemical compounds etc, but why? It would have only caused more confusion.
I watched him do a full scientific presentation to some police officers on ways meth was being manufactured and I assure you that they all left (myself included) feeling like we were more ignorant than your local toothless meth cooker. No one knew anymore after the meeting than before and many of them knew even less. They were better off just asking the people they arrested, and that's how they figured it out. Not by the reactions taking place or any of the compounds. But by the short version. Sometimes the short hand version is all one needs or even wants to know.

 

[Olon]

Very disappointing statement to hear from an engineering student (said as a career engineer myself. Study internal ballistics for powder gasification, combustion, and bore pressure curves for about 20min some day and you will see the fallacy in your statement.

Powder is gasified early in the bore, then the gases are combusted as the bullet travels.

The combustion reaction converts a relatively high density solid powder into a high pressure gaseous mixture. The “instant the powder is burnt,” the bore behind the bullet is full of high pressure gases - as you’d realize as an engineering student, pressure is a force applied over an area, and in our particular physical system, the bullet will remain accelerating (albeit variable acceleration) any time the force behind the bullet exceeds the friction of the bearing surface on the bore wall plus the relatively infinitesimally small inertia of the air in front of the bullet. So no, the moment the powder is burnt, the bullet does not start slowing down. The powder is burnt, it remains a high pressure system on one side, unbalanced, such a net outward force remains until the expansion is satisfied. PV= znRT at that point....

ETA: so your free body scribbling isn’t necessarily incorrect, but your basic understanding of when the propelling force of the system declines is the problem.

Yeah I realized that after I drew the fbd. Pressure is still around after the powder is burnt bc theres still gas expanding obviously. As that volume increases the pressure also does and at a certain point the differential of the acceleration is 0 and after that it might slow down at some point. Thanks for holding me accountable though.

Didn't think it'd be honest for me to go back and edit my response so I posted the actual fbd as a revision. Should have recanted the original though.

 

[Varminterror]

As far as the Condensed version of the events taking place by an engineer...

Explaining complex systems to “simple people” is an art form. Some of us get really good at talking down to people about technical systems without making them feel that way, and can make millions of dollars for it, and billions of dollars for our companies. Unfortunately a lot of engineers don’t have those people skills to communicate effectively, so as you noted - a lot of talking happens, but no learning. Sometimes that’s a bad mouth, sometimes it’s a bad ear, sometimes both.

What is exceptionally frustrating from that position, however, is when someone has heard, read, and largely misinterpreted pseudoscience or “half science” then propagates it to support flawed theory and mislead (inadvertently or otherwise) those other folks listening who don’t know any better. From where I sit, it’s worse than passing around the clap.

 

[ATLDave]

Explaining complex systems to “simple people” is an art form. Some of us get really good at talking down to people about technical systems without making them feel that way, and can make millions of dollars for it, and billions of dollars for our companies. Unfortunately a lot of engineers don’t have those people skills to communicate effectively,

 

[Olon]

Sorry guys, my initial post about slowing down as soon as powder is burnt was wrong. I acknowledge that. What I tried to scribble out is what I see as a proper application of the principles behind the process in a relatively simple way. Should've done that in the first place but didn't. Lesson learned.

I think a lot of folks took what I was saying as condescending and that wasn't my intention. I apologize if that's the perceived tone of it. I'm here to learn just like everybody else. I can solve a differential equation but I sure as hell don't know how to bed a rifle nor am I even a good enough marksman to reap the benefits.

My intention was to put the problem down as I understand it and that's what you've got here. Both before and after I really started thinking about the scientific reasoning. That's the best I can do and all I know how.

 

[Varminterror]

Buy Quickload, take your laptop into your closet on a Friday night, light a candle, and nerd out for a while. You’ll likely be late to class on Monday. Engineers can achieve greatness by standing on the shoulders of giants - the math is all done for you in QL to get a very clear picture of when your bore drag would exceed your bore pressure and result in a net deceleration force...

 

[jmorris]

But ive personally dealt with this on two occasions, where the barrel length wasn't optimal for the ammunition.

I was just a kid when I experienced the phenomenon for the first time, ammunition that ran supersonic out of an 18” barrel was noticeably louder than when I fired it from a 24” barreled single shot becuse it exited subsonic.