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Author: Subject: Gun design
OneEyedPyro
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[*] posted on 18-9-2016 at 00:41


Quote: Originally posted by chemrox  
Is it possible to make a .40 subsonic load that would still make enough pressure to operate the pistol recoil-loading mechanism.


Plenty of .40 S&W ammo is subsonic to begin with.

Basically any automatic handgun cartridge can be made to cycle properly at subsonic velocities if loaded with a heavy enough bullet and a suitable powder, there are some exceptions such as bottlenecked cartidges that are naturally high velocity like the 5.7X28 but slight modifications to a firearm like a weaker recoil spring can make underpowered ammo function well.

[Edited on 18-9-2016 by OneEyedPyro]
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[*] posted on 19-9-2016 at 04:33


Quote: Originally posted by chemrox  
Is it possible to make a .40 subsonic load that would still make enough pressure to operate the pistol recoil-loading mechanism. You can tell from the question I know little about ballistics and firearm terminology. (I do know what a lower receiver is) ;^)
CRX

an added question- I just did a search for "wetted NC". Wetted NC is used for ammunition. What is "properly wetted nitrocellulose?" How is it made from NC?

[Edited on 6-9-2016 by chemrox]


Yes, generally the subsonics for .40 and 9mm pistols/sub guns are heavier than the supersonic ammo for the purpose of ensureing postive function, as well as providing higher sectional density ro increase penetration at lower than usual velocity. There are also booster devices sometimes used on delayed blowback pistol barrels to help ensure the added weight of the silencer does not prevent normal function.

Wetted NC has not less than 25% water, per US DOT shipping regulations- that is, you should have 1 gram of water mixed in for every 3 grams of NC for it to be considered non explosive in shipping or storage.




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[*] posted on 22-9-2016 at 06:46


Quote: Originally posted by Hennig Brand  

...

2. A large portion of the energy released when a propellant burns is used up accelerating the combustion products, not just the bullet.

...

7. The velocity of the bullet/projectile can never be more than the escape velocity of the gases produced.
...



2. Depends on what "large" means to you. At the moment the bullet exits the barrel, only the gas immediately in contact with it is going the same speed. The gas at the wall of the chamber is completely motionless, half-way down the barrel it is moving at half the exit velocity , and has one quarter of the kinetic energy per gram as the gas at the front. The pressure is everywhere the same in the barrel, and temperature and density are pretty close to even also. Since the propellant mass is smaller than the bullet (1.9 g vs 4 g for the SS109 NATO round for example) we can immediately tell that the energy accelerating the propellant is at least proportionally less (< 1.9/5.9 of the total energy), and by applying a little integral calculus we find that the average kinetic energy per gram of propellant in the barrel is only 1/3 of that of the bullet, so for the SS109 it possesses only 13.7% of the total KE.

7. Yes, indeed we can say that at the moment the bullet base clears the barrel (neglecting the boat-tail, if any), they are exactly the same for the infinitesimal quantity of gas adjacent to the bullet.(Note that as soon as the bullet clears the barrel the gas in the barrel starts to expand much faster as it is no longer confined, and part of it will exit the barrel faster than the bullet did, a small part won't exit the barrel at all)


Quote: Originally posted by nitro-genes  

...
I fired a rifle at a commercial range few times, and the barrel does tend to get pretty warm after a few shots, is this mostly from friction energy loss or heat conductance?. For a combustion engine efficiency is about 35%, has anyone ever calculated this for a rifle? Could you seriously reduce the pressure exponent for NC by pressing (can it be deadpressed?) it as one solid block using hydraulic press, than having a very large chamber and a small extremely long barrel (enough to allow all propellant to be burned inside the barrel) would you find max velocity possible for NC using rifle?

[Edited on 26-6-2016 by nitro-genes]


Mostly heat conductance. Bullet friction in the barrel is much smaller than that (1/3 or so).

If is very easy to calculate the energy efficiency of a gun. The kinetic energy of a bullet is calculated from KE=0.5*MV^2, the energy content of the propellant is the weight of the propellant times its energy content (about 5400 J/g for modern propellants). The efficiency of guns ranges from about 10% to a high of 37% or thereabouts, so yes they can be very efficient heat engines. The longer the barrel the more efficient the gun is, so rifles are more efficient than hand guns, the low end are very short barrel hand guns. But guns aren't designed around energy efficiency, really, so it is possible to push it even higher with a very long barrel and an optimized bullet/propellant mass ratio.

Also, all of the propellant is burned inside the barrel in all guns of competent modern design (extremely short barrel handguns might be an exception).

The maximum possible bullet velocity would be with a very, very long barrel and a very light bullet, one much lighter than the powder loading.

The limiting velocity is actually much higher than the speed of sound in the burned propellant (although it is related to it). Much of the energy in the compressed driving gas exists as internal energy (rotational and vibrational energy of molecules), as it expands this energy gets converted to translational energy (i.e. motion).

This limiting velocity is related to the speed of sound (c) by the relationship:

u = 2c/(gamma - 1)

where u is the velocity and gamma is the specific heat ratio of the gas.

For molecular gases gamma is usually in the range of 1.1 to 1.5. For normal air it is 1.4. According to my copy of the US Army Pamphlet
"Principles of Explosive Behavior" gamma for the hot combustion gases of Comp B is 1.25. Nitrocellulose has similar elemental composition and
energy content so it should be much the same.

Thus u has a limiting value of eight times the speed of sound in the same combustion gases. The speed of sound in hot dense combustion gases will be much higher than the 330 m/sec of air at 0 C (273 K). Since the combusion gases are something like a factor of ten hotter than air we could
guesstimate a ballpark value of c at over 1 km/sec (square root of 10 times 330 m/sec), giving the limiting velocity the impressive value of
circa 8 km/sec!

[Edited on 22-9-2016 by careysub]




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[*] posted on 22-9-2016 at 11:16


Nitpick: I think 3500-4500J/g is more correct for small arms propellants. These numbers come from VihtaVuori and should be representative for most powders.



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[*] posted on 22-9-2016 at 12:21


That was supposed to be 5400 kJ/g actually (I have corrected), and was based on an estimate I made making from a composition of 1/3 NG and 2/3 NC (13.35% N) using the UCRL-52997 published experimental energy of explosion.

It did not include inert materials.



[Edited on 22-9-2016 by careysub]




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[*] posted on 22-9-2016 at 13:47


I don't think any modern powders use anywhere near that much NG. VihtaVuoris N550 is double base and is listed to 4250J/g.



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[*] posted on 22-9-2016 at 14:28


I found a composition for Olin WC 844, used in the M-16 round (at some time), which is:
Nitrogen in nitrocellulose 13.05–13.20
Graphite 0.4
Sodium sulfate 0.5
Nitrogylcerin 8.0–11.0
Diphenylamine 0.75–1.50
Dibutylphthalate 3.0–6.0
Nitrocellulose ~84%

This gives a high value of 4850*0.84 + 0.11*6650 = ~4805 kJ/g

(Actual energy release in a burned propellant may be different from LLNL handbook values.)

[Edited on 22-9-2016 by careysub]




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[*] posted on 23-9-2016 at 00:36


VV's N100-series (rifle powders) are single base, single perforated, surface treated powders. The fastest is N110 which has an energy content of 4000J/g. The slowest (N165) only has 3600J/g. The same pattern can be seen with the N500-series (double base, single perforated powders, max 25% NG). The fastest is N540 with 4250J/g, the slowest is N560 with 4000J/g. So the amount of burn rate retarders seems to have a huge effect on energy content.
The pistol powders (N300-series) are porous single-base powders, meaning the burn rate is regulated by the amount of surface porosity rather than by adding inhibitors. All of these are 4200J/g.




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