External Ballistics Experiment

Its204fast

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This has been a thought I've had for a while.

How much does the density of the air in front of the bullet affect velocity, trajectory, and accuracy?

If you could reduce the air density in the barrel in front of the bullet would it make a measurable difference? I feel it would.

I want to work up a load, get the SD as low as possible, fire a group at a target at 100 yards for the control, then another group with a Helium balloon over the barrel purging Helium down the barrel. The idea is that Helium is lighter than air and will reduce pressure in front of the bullet theoretically allowing the bullet to move faster.

I plan to start with 100 rounds of virgin Hornady brass, CCI BR-4 Primers, Benchmark Powder, 26Gr Barnes Varmint Grenade.
Rem 700 chambered in .204 Ruger with apx. 1000 rounds through it.

Is this something someone has attempted or or does anyone have any thoughts on this not working?

Some questions and concerns I have are:
Is my reloading ability and equipment capable of making loads as consistent as necessary for best results? I Full length size (as opposed to neck sizing) and I don't have the ability to turn necks.
Is my gun up to the task?
Could the Helium cause a catastrophic event with the gun?
Is 100 yards enough to see a change in POI due to the (likely) minor change in velocity
 

Trnelson

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Man, you are pretty deep in the weeds. Fun! At least I’m not the only one that thinks about this kind of stuff!!!

I’ll limit my response, for the sake of brevity, to just the internal ballistics at a fixed temperature. Simply stated, Atmospheric pressure, expressed in mass per unit of volume does effect velocity we can easily prove that. Upon exiting the barrel the projectile is continuously decelerating due to gravity, and the density of the atmosphere that it passes through among other things.

Here comes the sciency part that you can just skip
over, like we all did in high school science class... The specific density of Helium in its gaseous form is 0.0103 at 70•f. A standard atmosphere is expressed as a density of 1.225 Kg/m^3 at sea level and 15•C. In similar terms, atmosphere is 29.0 g/mol and helium gas is 4.0 g/mol. Helium is 7.25 times less dense, in terms of mass per unit volume, than a standard atmosphere.

Given that we can demonstrate that a bullet Is accelerated to a higher velocity simply by firing it in a less dense atmosphere, it only stands to reason that by further artificially manipulating the density of the initial medium that the bullet is accelerated within you would be able to see measurable increases in muzzle velocity. How much, would you gain in terms of velocity? IDK.

That being said there are several issues that would Complicate utilizing that setup in the real world. Primarily sealing the barrel at each end so the gas wouldn’t leak out. A simple ballon would likely solve the issue at the muzzle end. At the breech it would be more complex as you need to leave some room for the brass to obturate upon firing, unless you want to manually remove each spent piece. Also you would have to plug the safety hole in the barrel that vents gas in the event of a Cartridge failure or muzzle obstruction. Maybe a ballon or a delicate plastic plug that would seal it while the pressure was low and be pierced if the pressure exceeds some amount. Then how do you account for how temperature effects density of the helium? If it’s 70• at camp at 3,000’ MSL and you go up the mountain to where it is 9,000’ MSL and 40• or if you seal the helium in at 9,000’ MSL and 40• what happens when you go back down to camp at 3,000’ MSL and 70•
IMO, this is fun to think about, but I just don’t believe that the juice is worth the squeeze.
 

speedengineer

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Interesting idea! Though, I think the difference in mass in front of the bullet will be quite negligible.

Considering a 0.204" diameter bore, 24" long in front of the bullet:
barrel air mass.PNG


That shows that by replacing air with helium, the mass of gas in front of the bullet will be reduced by 0.208 grains. This is less than 1% of your specified bullet mass. For a larger cartridge with a heavier bullet, the percent reduction would be even less significant.

Good luck!
 

Mikecr

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Then there is the matter of powder response to less forward resistance (due to lower volume density).
Lower forward resistance would lower pressure a bit which would counter potential increase in muzzle velocity.
You end up with practically no change.

I see this matter with use of tungsten disulfide in the bore/coated bullets.
This is the slipperiest dry film lube, and yet it does not affect MV.
 

Muddyboots

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trnelson: first thought I had as well. Create a vacuum for the rifle to sit in and then fire the round. But would you have ignition in the case without oxygen? Or how about shooting it from a vehicle traveling 100mph to gain 146 fps? Ok back to my Jim Beam.
 

P7M13

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  1. Dont quit your day job.
  2. Any density variation is negligible, minuscule relative to the resistance forcing the bullet down the barrel.
  3. Assuming the balloon maintains some pressure and its position on the barrel, the resistance penetrating the latex will eliminate any advantage by the density reduction.
  4. If you consider the density of air inside a 140°F barrel after a few shots, you already measure this to a lesser extent. Do your chrony numbers indicate this?
 

Tommytrees

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trnelson: first thought I had as well. Create a vacuum for the rifle to sit in and then fire the round. But would you have ignition in the case without oxygen? Or how about shooting it from a vehicle traveling 100mph to gain 146 fps? Ok back to my Jim Beam.
I second Muddyboots suggestion. I have no idea about the scientific stuff but agree with the idea of 2 fingers of Bourbon (Knob Creek) but its too early.
OP good luck with your experiment. Be safe be well :)
 

P7M13

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Create a vacuum for the rifle to sit in and then fire the round. But would you have ignition in the case without oxygen?
gunpowder (smokeless variety and black) has it's own oxygen source.

 

Its204fast

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Man, you are pretty deep in the weeds. Fun! At least I’m not the only one that thinks about this kind of stuff!!!

I’ll limit my response, for the sake of brevity, to just the internal ballistics at a fixed temperature. Simply stated, Atmospheric pressure, expressed in mass per unit of volume does effect velocity we can easily prove that. Upon exiting the barrel the projectile is continuously decelerating due to gravity, and the density of the atmosphere that it passes through among other things.

Here comes the sciency part that you can just skip
over, like we all did in high school science class... The specific density of Helium in its gaseous form is 0.0103 at 70•f. A standard atmosphere is expressed as a density of 1.225 Kg/m^3 at sea level and 15•C. In similar terms, atmosphere is 29.0 g/mol and helium gas is 4.0 g/mol. Helium is 7.25 times less dense, in terms of mass per unit volume, than a standard atmosphere.

Given that we can demonstrate that a bullet Is accelerated to a higher velocity simply by firing it in a less dense atmosphere, it only stands to reason that by further artificially manipulating the density of the initial medium that the bullet is accelerated within you would be able to see measurable increases in muzzle velocity. How much, would you gain in terms of velocity? IDK.

That being said there are several issues that would Complicate utilizing that setup in the real world. Primarily sealing the barrel at each end so the gas wouldn’t leak out. A simple ballon would likely solve the issue at the muzzle end. At the breech it would be more complex as you need to leave some room for the brass to obturate upon firing, unless you want to manually remove each spent piece. Also you would have to plug the safety hole in the barrel that vents gas in the event of a Cartridge failure or muzzle obstruction. Maybe a ballon or a delicate plastic plug that would seal it while the pressure was low and be pierced if the pressure exceeds some amount. Then how do you account for how temperature effects density of the helium? If it’s 70• at camp at 3,000’ MSL and you go up the mountain to where it is 9,000’ MSL and 40• or if you seal the helium in at 9,000’ MSL and 40• what happens when you go back down to camp at 3,000’ MSL and 70•
IMO, this is fun to think about, but I just don’t believe that the juice is worth the squeeze.

This is the type of response I was looking for.

As far as sealing the breach, I'm not concerned, the balloon will slowly deflate (forcing the ambient air out of the barrel and chamber) before I fire, next shot will require a new balloon.

Responding to your statement about using a vacuum, I thought about that but it's not within my budget to consider.
 

Its204fast

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Then there is the matter of powder response to less forward resistance (due to lower volume density).
Lower forward resistance would lower pressure a bit which would counter potential increase in muzzle velocity.
You end up with practically no change.

I see this matter with use of tungsten disulfide in the bore/coated bullets.
This is the slipperiest dry film lube, and yet it does not affect MV.

like 2 loads that are the same except on has less neck tension that the other. the tighter neck is going to require higher pressure to expel it from the case. Following Newtons third law of motion.
 

nmbarta

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There is another way to look at this,
You can get a rough idea of the maximum possible gains by looking at your ballistics calculator.
Take 55 gr berger @ 3000fps, and set your start range at zero and your step size at one.
What you'll find is, that bullet slowing down between 2 and 3 fps per yard. This reduction is speed is mostly due to atmospheric resistance. So based on that, 3fps in a 36" barrel would (in theory) be the maximum possible gain, but that would require the bullet to be at 3000fps all the way through the barrel, which it is not. You also have to consider, what percentage air resistance has on the overall acceleration of the bullet (likely a small percentage in comparison to friction and bullet engraving) Though, there probably is some change, I would bet that the actual measurable difference between firing a rifle at sea level, and firing a rifle in a vacuum would be a small fraction of 1 fps. This is obviously not a big enough difference for us to measure.
Fun stuff to think about, but I'm quite sure you won't find any measurable difference.
 

Radman

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Nov 23, 2019
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TN
trnelson: first thought I had as well. Create a vacuum for the rifle to sit in and then fire the round. But would you have ignition in the case without oxygen? Or how about shooting it from a vehicle traveling 100mph to gain 146 fps? Ok back to my Jim Beam.
Been think the same thing. Since the He would purge all O I'm thinking that you may lose the follow-up "push" down the barrel after the initial internal bullet explosion. May be a negligible factor. (I still cant figure out how a translucent plastic pulley, that I can twist with my hand, can handle steel cables bearing 350 tons AND swing around!)
 
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