9mm bullets, guns, and Physics question...

[Cuz]

Can someone help me wrap my head around this.

 

I have a 124gr 9mm load that chronos at 1024 FPS from my Glock 34 and produces a 127 power factor.  I can reduce the powder charge to get a new load that chronos at 970 FPS for a power factor of 120.  This new "reduced power" load would be a softer shooting "lighter" load.

 

Now, if I took my original load that chrono'd at 1024 FPS in my Glock 34, and shoot it in my Glock 17, it chronos at 970 FPS for a 120 power factor.  Would this round then feel "lighter" and be a softer shooting round when shot in the G17?

 

I've tried shooting them both at the range and I can't really tell the difference, but ultimately, there has to be.  The guns are almost the same weight, the G34 is 0.8 oz heavier.

 

Some of you physics gurus must know how to figure this out...

 

Thanks,

-Cuz.

 

[4n2t0]

Power factor = muzzle velocity x bullet weight ÷ 1000. (note firearm weight isn't even a factor [or charge weight for that matter]).

 

Without going to deep into it a heavier gun will have less felt recoil than a lighter one, all else being equal.

 

http://www.shooterscalculator.com/recoil-calculator.php

Edited August 3, 2020 by 4n2t0

[Umbrarian]

25 minutes ago, Cuz said:

Can someone help me wrap my head around this.

 

I have a 124gr 9mm load that chronos at 1024 FPS from my Glock 34 and produces a 127 power factor.  I can reduce the powder charge to get a new load that chronos at 970 FPS for a power factor of 120.  This new "reduced power" load would be a softer shooting "lighter" load.

 

Now, if I took my original load that chrono'd at 1024 FPS in my Glock 34, and shoot it in my Glock 17, it chronos at 970 FPS for a 120 power factor.  Would this round then feel "lighter" and be a softer shooting round when shot in the G17?

 

I've tried shooting them both at the range and I can't really tell the difference, but ultimately, there has to be.  The guns are almost the same weight, the G34 is 0.8 oz heavier.

 

Some of you physics gurus must know how to figure this out...

 

Thanks,

-Cuz.

 

No.

 

From a physics point of view (f=ma), the maximum f has already been achieved before the bullet is halfway down the barrel. So shortening the BBL will lower the PF, but not the force.

 

As stated above, heavier gun has less felt recoil (also an f=ma situation, because the gun has more mass now.)

[superdude]

Recoil force has 4 components; bullet weight, bullet speed, gun weight, powder weight. 

 

Plugging those into a recoil formula like the one 4n2t0 posted will tell you how much recoil force is generated for each gun. 

 

Felt recoil is subjective and the subject of a different discussion. 

[Wesquire]

Given the same weight, the longer barrel/faster bullet gun will have more recoil because momentum is conserved. 
 

3 hours ago, Umbrarian said:

From a physics point of view (f=ma), the maximum f has already been achieved before the bullet is halfway down the barrel. So shortening the BBL will lower the PF, but not the force.

This is not true because the bullet is obviously still accelerating, otherwise the muzzle velocity would be identical. And beyond that, force is not what is relevant here. 

Edited August 3, 2020 by Wesquire

[Umbrarian]

17 hours ago, Wesquire said:

Given the same weight, the longer barrel/faster bullet gun will have more recoil because momentum is conserved. 
 

This is not true because the bullet is obviously still accelerating, otherwise the muzzle velocity would be identical. And beyond that, force is not what is relevant here. 

 

Yes it is true.

 

Peak acceleration, and thus peak force, is achieved within the first few centimeters of bullet movement. After that, pressure is dropping fast, and while it is still accelerating, it is accelerating at a decreasing rate.

 

[Wesquire]

13 minutes ago, Umbrarian said:

 

Yes it is true.

 

Peak acceleration, and thus peak force, is achieved within the first few centimeters of bullet movement. After that, pressure is dropping fast, and while it is still accelerating, it is accelerating at a decreasing rate.

 

 

Except you didn't just say peak force.

 

"So shortening the BBL will lower the PF, but not the force."

[Cuz]

No.
 
From a physics point of view (f=ma), the maximum f has already been achieved before the bullet is halfway down the barrel. So shortening the BBL will lower the PF, but not the force.
 
As stated above, heavier gun has less felt recoil (also an f=ma situation, because the gun has more mass now.)

If the maximum force is applied before the bullet is half way down the barrel, wouldn’t the longer barrel produce lower velocities?

The bullet has to still be increasing speed during the last half inch of the longer barrel.


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[motosapiens]

pf is related to recoil, but is not the same as recoil. a longer barrel will both increase pf and slightly reduce muzzle-flip and/or perceived recoil.

 

I think muzzle flip is probably a more important component of perceived recoil than just the plain old measured recoil force.

[Wesquire]

38 minutes ago, Cuz said:

If the maximum force is applied before the bullet is half way down the barrel, wouldn’t the longer barrel produce lower velocities?

The bullet has to still be increasing speed during the last half inch of the longer barrel.


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No, he is right that maximum force happens relatively early. It is just that total force is still higher with a longer barrel (assuming everything else is constant).

[Umbrarian]

58 minutes ago, Cuz said:

If the maximum force is applied before the bullet is half way down the barrel, wouldn’t the longer barrel produce lower velocities?

The bullet has to still be increasing speed during the last half inch of the longer barrel.


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No.

 

Think of it like a car. You slam the g-a-s p-e-d-a-l, you get pinned back in your seat, for most cars, that is when peak acceleration occurs. But the car is still accelerating as you pass 50/75/100/etc, , but not as much as in the first few seconds. Same thing is going on in the barrel.

 

 

 

 

 

Edited August 4, 2020 by Umbrarian

[Umbrarian]

53 minutes ago, Cuz said:

The bullet has to still be increasing speed during the last half inch of the longer barrel.
 

Here is an interesting chart showing velocities vs BBL length:

 

http://ballisticsbytheinch.com/9luger.html

[Umbrarian]

15 minutes ago, Wesquire said:

 

It is just that total force is still higher with a longer barrel (assuming everything else is constant).

 

Since f=ma, and m is constant, f cannot get any higher then when acceleration is at peak. Since peak acceleration occurs early, Longer BBL lengths after that do not matter wrt to f.

[Wesquire]

2 minutes ago, Umbrarian said:

 

Since f=ma, and m is constant, f cannot get any higher then when acceleration is at peak. Since peak acceleration occurs early, Longer BBL lengths after that do not matter wrt to f.

 

Except peak force is just one aspect. The total force still goes up after peak force. And again, force isn't even the most relevant metric here, momentum is. 

[Umbrarian]

5 minutes ago, Wesquire said:

 

Except peak force is just one aspect. The total force still goes up after peak force. And again, force isn't even the most relevant metric here, momentum is. 

 

 f= ma, m is constant. Thus when a is peak, f is peak. It cannot go up after reaching peak, as that then would be peak.

 

I do not see how momentum matters. You have Newtons' Second Law -> f=ma, and you have his Third Law -> equal and opposite. So the f number is what it is. Now felt recoil, that is a different and things like gun mass, spring weight, force duration, etc. all factor in.

[Wesquire]

14 minutes ago, Umbrarian said:

 

 f= ma, m is constant. Thus when a is peak, f is peak. It cannot go up after reaching peak, as that then would be peak.

 

I do not see how momentum matters. You have Newtons' Second Law -> f=ma, and you have his Third Law -> equal and opposite. So the f number is what it is. Now felt recoil, that is a different and things like gun mass, spring weight, force duration, etc. all factor in.

 

You are misunderstanding this. For example, let's say a 100gr bullet gets up to 750fps when it is half way down the barrel. Then in the other half it still accelerates to 1000fps. The peak happened in the first half, but the TOTAL force is still higher by the time the bullet exits the muzzle than it was at the half way point.

 

Force is redundant and is derived by the rate of change in momentum. It is also not conserved like momentum is. Newton's second law is about the rate of change in momentum. Additionally, Newton's second has also been supplanted by the momentum principle  because Newton's second breaks down when an object approaches the speed of light.

[superdude]

Here's the numbers when plugged into a recoil formula. (using 0 for powder weight, but any value could be used as it is simply a constant.) Gun weight values are gun with empty magazine from Glock website. 

 

G34     gun weight 25.93 oz     1024 fps      recoil force = 3.16 ft lbs

 

G17     gun weight 24.87 oz     970 fps      recoil force = 2.95 ft lbs

 

 

[Umbrarian]

11 minutes ago, superdude said:

Here's the numbers when plugged into a recoil formula. (using 0 for powder weight, but any value could be used as it is simply a constant.) Gun weight values are gun with empty magazine from Glock website. 

 

G34     gun weight 25.93 oz     1024 fps      recoil force = 3.16 ft lbs

 

G17     gun weight 24.87 oz     970 fps      recoil force = 2.95 ft lbs

 

 

 

Yes. They are using the fps to get an average acceleration over distance of BBL, since you cannot submit the actual peak acceleration.

[Umbrarian]

15 minutes ago, Wesquire said:

 

You are misunderstanding this. For example, let's say a 100gr bullet gets up to 750fps when it is half way down the barrel. Then in the other half it still accelerates to 1000fps. The peak happened in the first half, but the TOTAL force is still higher by the time the bullet exits the muzzle than it was at the half way point.

 

Actually I understand like a PhD.

 

What you are describing is felt recoil. Actual recoil is an instantaneous component calculated from when a is peak.

[Wesquire]

This isn't going anywhere. Recoil calculators are readily available online if anyone is actually curious.

[MikeBurgess]

real physics answer is all the above are wrong because the barrel and slide assembly move, so your felt recoil is a combination of the recoil spring pushing the gun back into your hand and the barrel stopping on the locking block and the slide bottoming out on the frame and the recoil spring pushing the slide forward and the slide loosing energy as it strips a round from the magazine and the barrel going into battery and stopping the slide. 

 

the chances that you could fire a round from one gun with its combination of springs and weights and losses and then fire a round in a different gun with a different combination of the above and feel a meaningful difference that is directly related to a 5.5% change in PF is as close to zero as you can get

[Cuz]

I wish I had something meaningful to add, but I am learning a lot.

 

MikeB,  You can't say everything is different because I can use the exact same frame and recoil spring so the only difference between the G17 and G34 would be the slide and barrel.  And, I already know that I can't "feel" the difference.  But, that doesn't mean there isn't a difference.

 

I'm just trying to determine with math/physics whether the longer barrel/slide combo would produce a softer shooting result than the G17.  I don't know what the answer is, but in my mind I "think" that the G17 would be softer because of the lower velocity.  But, my wife will tell you that what I think is rarely correct.

 

Thanks to the rest of you who have chimed in, it seems like there are a couple of differing opinions which means that the answer may not be as clear as I wished.

 

[rocket99]

9mm recoil.....is this really a discussion? Jeez we have too much time on our hands LOL


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[Cuz]

9mm recoil.....is this really a discussion? Jeez we have too much time on our hands LOL


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Yup, first world problems...
It gives us the distraction we need.


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[belus]

On 8/3/2020 at 7:18 PM, Wesquire said:

Except peak force is just one aspect. The total force still goes up after peak force. And again, force isn't even the most relevant metric here, momentum is. 

 

I'm not a physicist or mechanical engineer, and never even took a dynamics class in college which would have given me the real tools to discuss this.

But aren't you confusing Force with Energy and Impulse, ie the integral of Force over Distance and Time respectively?

The calculator isn't helping because it's quoting numbers of "recoil force" with units of energy anyways.

 

On 8/4/2020 at 4:27 PM, MikeBurgess said:

real physics answer is all the above are wrong because the barrel and slide assembly move, so your felt recoil is a combination of the recoil spring pushing the gun back into your hand and the barrel stopping on the locking block and the slide bottoming out on the frame and the recoil spring pushing the slide forward and the slide loosing energy as it strips a round from the magazine and the barrel going into battery and stopping the slide. 

Here's a paper published in the Journal of Sports Engineering and Technology that looks at recoil a gas operated shotgun with and without the gas system enabled. The measurements were sensitive enough that he could see an extra shell shift in the magazine tube.
https://www.researchgate.net/publication/276247289_Effects_of_breech_bolt_movement_on_felt_recoil_of_a_gas-operated_semi-automatic_sporting_gun
 

Quote

Abstract
The recoil forces characteristic of shoulder-mounted firearms can create discomfort and distraction for the shooter. A modified version of a previously developed system for measuring felt recoil was adapted to approximate the force interaction between gun and shooter with the aid of high-speed imaging. The high-speed imaging was used to find a combination of damping materials for the measurement system that would approximate the gun dynamics of a particular shooter. The recoil measurement system was then used to characterize the recoil of a gas-operated semi-automatic shotgun, comparing the recoil to a fixed breech version, quantifying the effect of the bolt movement on recoil forces. It was found that the time scale of the bolt movement relative to the main recoil event was important to the recoil force profile, increasing its duration, and for the particular gun examined, peak recoil forces were reduced between 20% and 25% by the bolt movement.

 

 

On 8/4/2020 at 5:02 PM, Cuz said:

I'm just trying to determine with math/physics whether the longer barrel/slide combo would produce a softer shooting result than the G17.  I don't know what the answer is, but in my mind I "think" that the G17 would be softer because of the lower velocity.  But, my wife will tell you that what I think is rarely correct

My perception is that higher pressures (Force/Area) in my hand feel more violent. I found a reliable load for my guns that made PF and then put the biggest fattest grips I could comfortably get my hands around to spread that force over the widest area possible. The guns feel plenty "soft" now.