More Ballistic Trivia

[Pistolsmith]

We are shooting a 1911 in 9X23 Winchester caliber from a modified Ransom Rest. The cartridge is a maximum load, having 45,000 psi chamber pressure (factory loading) behind a jacketed 124 grain bullet with a standard ogive. This combination will leave the muzzle at approximately 400 feet per second faster than the speed of sound at sea level (our range is some 200 feet above sea level.) Pistol is equipped with a very effective 3 port compensator. Ransom Rest is adjusted so that at maximum deflection, sights align below the 10 ring of out target. Muzzle is elevated by pivoting upward until sights align perfectly on the X. Shots are fired pneumatically, via a tube with a bulb at both ends so that aim will not be disturbed by squeezing off a shot.
1. After a shot is fired, will the sights be aligned at a point above or below the 10 ring due to muzzle flip or gas escape through the comp ports?
2. When the bullet exits the barrel muzzle and travels the length of the compensator in free flight, exiting through a hole in the front of the comp that is .004" greater in diameter than the bullet diameter, will the bullet scrape at any point inside the exit hole? If it does, will the scrape be in line with the bore due to forward bullet travel, or will it be radial, due to bullet rotation, or both?
3. If we have placed the first chronograph screen exactly 5 feet from the barrel muzzle (not the front of the compensator), will the chronograph read the speed of sound or the bullet's velocity?
4. If I eject my brand new, forty some dollar 9X23 magazine and it bounces off the shooting bench and lands on the concrete range floor, would you have an urge to jump up and down on it?

 

[MrAcheson]

By "sea level" do you mean STP for the speed on sound? Since the speed of sound is only dependent on temperature an elevation is not sufficient to define it.

 

[Dave R]

Geez, Pistolsmith, the only question I know the answer to is #4. The answer is "no", since that would (further) damage the magazine.

 

[Blackhawk]

I'll take the last question.

No.

 

[Blackhawk]

Dave beat me too it while I was trying to figure out the other 3.

 

[Pistolsmith]

Dunno where you got the info on speed of sound, but I went to the ballsiitics research lab at Aberdeen Proving ground and they stated:
Sea Level Altitude
29.53" mercury baromentric pressure
59 degrees ambient temperature
7.8% relative humidity
0.751 lb./ft squared air density
Speed of sound 1120.27 fps
Decreases as altitude increases. (Air less dense and colder.)

 

[Dave R]

I'm having fun splitting hairs here....

The speed of sound through air is based only on density. Density can be affected by either altitude, or temperature, or both. That's why pilots talk about density altitude.

You can take off form Albuquerque when its 40 degrees, and your density altitude is X. You can take off from ALB when its 105 degrees and your density altitude is Y. Same altitude, different density, due to temerature.

When people talk about the speed of sound at different altitudes, they are holding temperature constant at some number.

 

[MrAcheson]

Ok so they are using Standard Temperature and Pressure (STP). The Speed of Sound is a temperature dependent phenomenon and can be defined by the square root of the product of the ideal gas constant, the ratio of the specific heat, and the absolute temperature. "Sea level" is actually only a pressure measurement unless you refer to some sort of atmospheric standard at "sea level".

Oh BTW how long is your compensator and should I assume a 1:16 twist rate?

 

[MrAcheson]

No when people talk about the speed of sound (SoS) at different altitudes they are refering to an atmospheric standard such as the US/ISA 1976 standard which is what most pilots use. Most pilots also don't worry about the speed of sound, they worry about air density because air density determines the lift produced by their wings.

In an ideal gas, density is given by temperature and pressure. SoS is given by only temperature.

 

[MrAcheson]

Never mind Pistolsmith I just did the calculations and that would have to be one long compensator. Ignoring any acceleration in the compensator and assuming that the barrel is horizontal and unmoving, the compensator would need to be about 5 feet long by my calculations.

So the answer to the first part of Number 2 is no. But if you had a squib and it did hit, the angle of the scratch on the bullet should be consistent with the angle of the rifling in your barrel. By your logic that means both radial and in line with the bore.

I think number 3 would be the bullet velocity since by 5ft the bullet should be outrunning the sound wave by a large margin. But I have very little understanding of cronographs in practice.

 

[Pistolsmith]

Excellent! However, we cannot assume that the barrel is horizontal. The Ransom Rest has a pivot and there is nothing to back it up but the diameter of the pivot axle. So, would the muzzle rise when the bullet breaks free of the case, since the line of the bore is above the anchor point on the grip frame, or would it dip due to pressure of the gas rising through the ports?
The comp is 3" long and rifling twist is between 1/12 and 1/14 (I could only approximate the pitch; maker does not specify and my wire brush may have been slipping radially.}

 

[cratz2]

No 3, probably the speed of the bullet which may be very close to the speed of sound.

Between the comp and the fact that there probably isn't tremendous forward muzzle blast, I'd think that you'd get an accurate reading. If you're looking at a 20" barrel screamin' fast 270 Win or something, it should be further than 5 feet out. With a comped gun under 1400 fps, 5 should probably be accurate. Try it at 5 feet and 10 feet . Should only see about a 20 fps difference if that.

 

[cratz2]

No 2... huh. Never really though about it. I've owned two comped guns and both were plenty accurate so I would have to guess that if the comp was designed and installed properly, it should have no contact with the bullet at all. If there was contact made and you recovered the bullet, it would probably be close to a straight line. Whaveter the twist rate is would be how much turn there was on the bullet itself. A 9x23 barrel for high velocity bullets should have about a 1 in 16" twist rate. So the over the 3/4" of bullet length, there should be less than 1/16 of an inch in difference between where the mark started and where it stopped.

 

[T.Stahl]

Well, I'm not exactly a rocket scientist, I'm an aeronautical engineer, so all I can say is that MrAcheson is correct.

The s.o.s. = sqrt ( kappa * R * T) = sqrt ( kappa * R_universal / molar mass * T )

with:

R = R_universal / molar mass
kappa = c_p / c_v
T = temp. in Kelvin

 

[MrAcheson]

Hehe yeah. I'm a mechanical engineer and I interviewed to be a rocket scientist, but I didn't want a job with the people who blew up the Challenger. I hope NASA has the common sense to look at retiring the shuttle fleet soon.

 

[Dave R]

IANAE (I am not an engineer) but thinking practically, the answers are:

1. The sights will be above the 10-ring. The ports only redirect a portion of the total force. Recoil gets all the force. So it'll rise less than a non-ported gun, but it'll rise.

2. The bullet had better not hit that compensator, or your accuracy will be lousy. If it doe scrape, it'll scrape radially because unstable spinning objects tend to move in a small cirsle around the point of flight. Right?

3. Already answered correctly by another poster. The chrono better read the bullet velocity, or take it back. Don't know if the chrono is sensitive enough to read moving gas.

4. No. Gotta protect those expensive mags.

 

[Wil Terry]

I ALWAYS WANTED TO BE AN ENGINEER.

Driving those trains looks like a hellovabunch of fun to me.

 

[Jeeper]

As another Mech. Engineer here:

Dont chrono's use light to measure the speed which would make it irrelevant if it was going faster or slower than the speed of sound.

The bullet wont scrape due to the fact that the acceleration of the gun upwards is miniscule compared to the acceleration of the bullet. In addition the bullet is accelerating upwards at the same rate as the gun until it leaves the barrell and is inside the comp.

 

[Pistolsmith]

My chronograph uses sky screens. THe instructions from Oehler warn about spacing the first screen at fifteen feet to avoid getting a reading of the speed of sound. I have placed them closer several times and it does read the speed of sound. Mine is an Oehler model 35.

 

[Jeeper]

What does a sky screen do?

I believe that the screen is a backdrop for the light. From what I have read the chrono sends out light from the bottom to the top of the screen. Then it measures the time that it takes the bullet to go between the screens and uses it to calculate the speed. The reason that they are supposed to be placed away from the muzzle is that particles of powder can escape and set it off from the muzzleblast. I can really imagine how it could measure a shock wave.

 

[Ryder]

Point of order -

The change in the speed of sound in air due to temperature and pressure variables is miniscule compared to the difference of the speed of sound in air-vs-solids.

If you consider fluids as a solid (due to it's being incompressible) then shouldn't the Rh be the most important variable in determining the speed of sound? The above discussions appear to have negated it. Probably since the Rh was so low, considering an 8% Rh that might not be too unreasonable. Atmosphere is compressible, but only to a certain degree. The relationship is not linear... An 8% level of humidity does not equal an 8% increase in the speed of sound. Some desert ya got there. STP dictates a 50% Rh around these parts.


Don't think I'd be stomping any expensive magazines. Would be kicking myself a few times though.

 

[T.Stahl]

I think with solids the s.o.s. is related to the Youngs-Modul. Waddayathink?

 

[MrAcheson]

As I understand it, some cronographs use the shock wave coming off the bullet to measure the speed of the bullet. Thats why you don't want the cronograph too close to the muzzle, the bullet will be overwhelmed by the muzzle blast and other muzzle effects, which travel at the speed of sound. Some cronographs may use light and others (like what the military now uses for tank target practice) do use radar.