External Ballistics Experiment

Paper boy

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Jul 18, 2019
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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
Try studing the effect air dencity has on supersonic jets. Has the exect same effect but you can visualy see it up close. In shooting, we call it "Ballistics" exactly the same!
 

Trnelson

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Its204fast: This is a fun experiment, and I would like to hear the findings. One thing not mentioned in the responses so far is compression. Gas has inertia, and the denser the gas, the greater the inertia. As the bullet moves down the barrel, the gas resident in the barrel will compress, and resistance will spike. This will have greater effect than the mass of the gas that must be moved. Helium will change the sonic velocity of the compression wave, and the accuracy node of your barrel. Also, rather than using a balloon, tap the gas safety port in the breech and install a threaded check valve (one-way valve) fitting, attach a soft plastic feed line from a helium cylinder and flow regulator, and send a low flow (<5 ltr/min) of helium down the barrel to displace the air. Do not put anything over the muzzle, just let the helium flow through. The flow regulator, fittings, hose and check valve will set you back about $120.

https://www.amazon.com/dp/B001C11BP4/?tag=lrhmag19-20

https://www.amazon.com/dp/B07RXLLVSQ/?tag=lrhmag19-20

I strongly recommend you tie a string to the trigger and fire the gun remotely, until you see what the effects are. I do not think this is particularly dangerous, but anytime you try something new, you get unexpected results or side-effects. In all cases, wear eye protection and use moderate loads. Good luck!
Good idea. Eliminate the ballon and simply displace the ambient atmosphere. The best solutions are usually really the simplest
 

Muddyboots

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What would happen if you cryogenically took the barrel down to Absolute zero? Eliminate friction? Bullet fracture?
 

ilc

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Sep 22, 2016
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Before all the gadgets and computers, the rule for most rifles was 1 MOA per 10 degrees up or down and 1 MOA for every 10 % change in humidity. It was far from perfect but at the time a 600 yard shot was considered a long shot and it seemed to work well.

It will be tough for you to get good information for the differences unless you make many outings. Historical data for many years would help if anyone has any. Hunt at sea level and then going to 8,000 to 11,000 feet with much lower humidity, we generally made our changes and then proofed them By shooting a target at the expected distance to see if adjustments needed to be made.

I would suggest that you establish a bench mark for testing under one set of conditions and then test the same loads at the longest distance you feel comfortable with and shoot at least 5 shots and use the center of these groups to establish the measurement center point.

The heavy air (humidity) and the lowest altitude always lowered trajectory from any other location.
so we had to adjust no mater where we went with different conditions. you will have to look for every change to get good results. More than once, one change canceled the effect of another so good data is the only way to get meaningful data. :)

Interested in your results.

J E CUSTOM
It is a common misconception that high humidity = higher density or heavier air.

High humidity air is physically less dense because the molecular weight of water is less than nitrogen or oxygen.

It feels heavy or dense to humans because of its effects on evaporative cooling (sweat).
 

Tulsa Reiner

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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!!!"

You guys are enjoying a mind experiment, which is what Einstein did to prove almost all of his theories. Keep it up and you might come up with a brilliant idea. Maybe even a Nobel prize in shooting!

BTW: isn't helium flammable? (remember the Hindenberg!) You might create a giant explosion at the end of your barrel, so wear an asbestos suit, or better yet, stand back 50 yards.
 

ScottB338

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Feb 3, 2020
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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!!!.....IMO, this is fun to think about, but I just don’t believe that the juice is worth the squeeze.
I read your whole explanation and thought that it was concise, well thought out and scientifically clear. But that last sentence had me LAUGHING OUT LOUD. Well done. One day I’d like to use your quote, given the right circumstances.
 

Its204fast

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May 20, 2020
Messages
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North Carolina
Its204fast: This is a fun experiment, and I would like to hear the findings. One thing not mentioned in the responses so far is compression. Gas has inertia, and the denser the gas, the greater the inertia. As the bullet moves down the barrel, the gas resident in the barrel will compress, and resistance will spike. This will have greater effect than the mass of the gas that must be moved. Helium will change the sonic velocity of the compression wave, and the accuracy node of your barrel. Also, rather than using a balloon, tap the gas safety port in the breech and install a threaded check valve (one-way valve) fitting, attach a soft plastic feed line from a helium cylinder and flow regulator, and send a low flow (<5 ltr/min) of helium down the barrel to displace the air. Do not put anything over the muzzle, just let the helium flow through. The flow regulator, fittings, hose and check valve will set you back about $120.

https://www.amazon.com/dp/B001C11BP4/?tag=lrhmag19-20

https://www.amazon.com/dp/B07RXLLVSQ/?tag=lrhmag19-20

I strongly recommend you tie a string to the trigger and fire the gun remotely, until you see what the effects are. I do not think this is particularly dangerous, but anytime you try something new, you get unexpected results or side-effects. In all cases, wear eye protection and use moderate loads. Good luck!
I had considered putting a tube free flowing beside the barrel, but didnt feel it would force the helium down the barrel. Someone also said the reduced air resistance in the barrel may cause an equally lower speed due to less pressure needed to expel the projectile.

not sure where you are saying put a check valve but I dont see where its needed.

Regarding the safety of it all, i have thought quite a bit about this and in theory i don't see where there SHOULD be any safety issues (I expect the chamber pressure to be reduced) unless the reduced pressure causes a detonation like an under charge could.
 

Its204fast

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Working to get more velocity at the barrel is good. but I have been thinking about tthe other end of the path. Design a bullet to make the transision from super-sonic to sub-sonic without destabalizing.
I'm not a physicist. Regarding transonic destabilization of a bullet, it would probably help to look at what a jet endures making the transition from sub sonic to super sonic and back, however, the jet has propulsion so I would expect some differences there. the APS rifle was designed to fire under water but there was different "ammo" for it, which was 5.66x120MM and only had a very short range somewhere around 100yds I believe. I would think the length of the proj was what contributed to it traveling as far as it did. where most bullets tumble the second it leaves the barrel in water. I feel certain (relating to ELD bullets) this is the trick for bullets passing from super to sub sonic and not destabilizing.
 

Trnelson

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Nebraska
I read your whole explanation and thought that it was concise, well thought out and scientifically clear. But that last sentence had me LAUGHING OUT LOUD. Well done. One day I’d like to use your quote, given the right circumstances.
Consider it your own.
 

Calamity

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Nov 23, 2019
Messages
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Georgetown, Texas
I had considered putting a tube free flowing beside the barrel, but didnt feel it would force the helium down the barrel. Someone also said the reduced air resistance in the barrel may cause an equally lower speed due to less pressure needed to expel the projectile.

not sure where you are saying put a check valve but I dont see where its needed.

Regarding the safety of it all, i have thought quite a bit about this and in theory i don't see where there SHOULD be any safety issues (I expect the chamber pressure to be reduced) unless the reduced pressure causes a detonation like an under charge could.
You may be confusing the static pressure in the open bore with the dynamic pressure generated by the cartridge. When the gun is fired, the pressure generated by the powder behind the bullet may be somewhat less, due to less resistance in front of the bullet, but I suspect that it can be less only if the bullet moves down the bore faster. The powder has a fixed burn rate and pressure rise time not effected by the gas in front of the bullet, except for the resistance of the bullet to move. Does that make sense? As for the check valve, it is to prevent the 60,000 psi chamber pressure when the cartidge goes off, from going back up the hose to the helium bottle. Just to avoid losing and eye or some other body part, maybe this should remain just a thought experiment.
 

Its204fast

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North Carolina
You may be confusing the static pressure in the open bore with the dynamic pressure generated by the cartridge. When the gun is fired, the pressure generated by the powder behind the bullet may be somewhat less, due to less resistance in front of the bullet, but I suspect that it can be less only if the bullet moves down the bore faster. The powder has a fixed burn rate and pressure rise time not effected by the gas in front of the bullet, except for the resistance of the bullet to move. Does that make sense? As for the check valve, it is to prevent the 60,000 psi chamber pressure when the cartidge goes off, from going back up the hose to the helium bottle. Just to avoid losing and eye or some other body part, maybe this should remain just a thought experiment.
Makes sense. I may attempt this after deer season. As scientific as possible considering the equipment I have to work with. I would like to use a LabRadar, but I'm probably going to use my .204 and the LabRadar won't read it (its over 4000fps.) so if I can get the LabRadar, I will use a 7MM RM, if not i will use a Chrony and the .204.
 

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