Help me understand power factor divisions.

[KFP]

I think it would be good to see some local tests on the Dashprod regarding Power Factor as it would be quite a bummer rushing out to buy one and then not able to achieve the required factor.

Hi Corne - the tests were done locally by uncle Roger with local milspec ammo.

Maybe you should ask your boet to get hold of one again and run some rounds over the chrono confused0083

[Corné]

Do you think I am going to risk my chrono..... hpy71

This is quite a concern to me personally as I am contenplating a Dashprod for IPSC 3 gun shooting.

[Armed Preacher]

It has nothing to do with twist rate.

Hi SSP

SSP,I beg to differ. The rate of twist does affect the the muzzle energy in that "excessive spinning causes a reduction in the lateral kinetic energy of a projectile, thereby reducing its destructive power (the energy instead becomes rotational kinetic energy)" (quoted from [ame]http://en.wikipedia.org/wiki/Rifling[/ame])
In simple terms, the greater rate of twist requires more energy to turn the bullet with consequently less energy available to push the bullet down the barrel for any given cartridge.

Rate of twist is generally seen in the light of bullet stabilisation and rarely does anyone equate twist with the potential to alter muzzle velocity.

[vaughan]

It has nothing to do with twist rate.

Hi SSP

SSP,I beg to differ. The rate of twist does affect the the muzzle energy in that "excessive spinning causes a reduction in the lateral kinetic energy of a projectile, thereby reducing its destructive power (the energy instead becomes rotational kinetic energy)" (quoted from [ame]http://en.wikipedia.org/wiki/Rifling[/ame])
In simple terms, the greater rate of twist requires more energy to turn the bullet with consequently less energy available to push the bullet down the barrel for any given cartridge.

Rate of twist is generally seen in the light of bullet stabilisation and rarely does anyone equate twist with the potential to alter muzzle velocity.

yes this makes alot of sense to me! have to agree here

[SSP]

Ah. Wikipedia. The online "encyclopedia" who anyone can add to regardless of qualification. Sorry but I don't trust the source. It may be true, but its unverified.

Quoted from the same article "This article does not cite any references or sources".

[Halicarna]

I have to agree with Armed preacher.

The rifling definately has an effect on the velocity. The rifling in the barrel is designed to induce an angular momentum which creates a gyroscopic effect keeping the bullet stable throughout it flight thereby reducing drag and increasing the distance the bullet will travel, also increasing the accuracy. The theory is that the faster it spins the more accurate it will be.

However, there is a payoff. The bullet will lose velocity if the pitch is too small. This will incur angular acceleration requiring a force in an angular direction, which will reduce the net force being applied to the bullet for the purposes of accelerating the bullet along its flight path. This would result in a rotational kinetic energy which is given as ½ mass times radius² times the angular acceleration² or ½.m.r².?². Thus, a low pitch would result in a high angular acceleration, which requires more energy. If a longer barrel is used and the number of rotations within the barrel are maintained it would increase the pitch resulting in a lower angular acceleration and still achieve the same angular velocity when the bullet leaves the barrel.

What I have just mentioned can be found in any Physics Textbook. The oneI used is "Physics for Scientists and Engineers with modern physics"

[Double_Tap]

Hi All

SSP is correct :-)

BACK TO BASICS
The Dashprod / LM6 struggle to achieve adequate velocities becuase they have SHORT barrels and the powders used by the manufacturers are intended for use in much longer barrels.

In relative terms a hunting barrel is typically 24" , LM 4 and Ruger Mini 14 +- 18" and Dasprod - I think 11".
So take a 8" 357 Magnum and compare it to a 2" or 3" snubbie.

Bullet twist affects the "spin rate" and that is a major part of the gyroscopic stability - which by the way also requires adequate bullet speed.
Any velocity loss due to energy conversion to spin energy is MINISCULE and it is required for bullet stability!

SUMMARY:
Issue is short barrel and powders designed for longer barrles

[ikor]

Indeed, the amount of velocity lost to spin a tiny bullet like a 55gr 5.56mm is so small as to be unmeasurable to most of us. Take 100 samples of each...20in. AR's with 1 in 12, 1 in 9 and 1 in 7 twists and fire the same LC M193 55gr ammo in each and you will quickly see that there is much more velocity difference from one barrel of identical twist rate to another than from a slower twist (1 in 12) to a faster one (1 in 7).

It is all about barrel length with the 5.56 / .223 cartridge, especially so if using FMJ bullets that depend on fragmentation as a wounding mechanism such as the old SA military ball or US M193 / M855. 20 inch barrels will usually give around 3250 to 3300 FPS MV with M193...16 in cuts that by around 200 FPS to about 3050 or so, and 11.5in cuts it further to around 2900, and this ammo is full bore mil spec stuff. So it is easy to see how the Dashprod could fail to make power factor with 55gr ammo loaded to less than 'WFO' power (Wide Freaking Open). If you have a source you might be better off to try the 75-77gr bullets as they do not lose so much velocity when barrel length is reduced. Black Hills 75gr runs around 2700 in a 20in, 2650 or so in 16in and 2575 or thereabouts in a 14.5in. I could not find chrono figures for it in an 11.5 in or shorter barrel but extrapolation would suggest it should run at least 2400 and probably more...easily making the minimum power factor of 150.

For some reason I was thinking the little Dashprod had only a 8 or 9in barrel...can anyone clarify that? A 55gr at 2900 makes 159 and a 75gr at 2400 makes 180 by my calculations...but it IS early here and I am only on my second cup of coffee ;D

[SSP]

Indeed, the amount of velocity lost to spin a tiny bullet like a 55gr 5.56mm is so small as to be unmeasurable to most of us. Take 100 samples of each...20in. AR's with 1 in 12, 1 in 9 and 1 in 7 twists and fire the same LC M193 55gr ammo in each and you will quickly see that there is much more velocity difference from one barrel of identical twist rate to another than from a slower twist (1 in 12) to a faster one (1 in 7).

It is all about barrel length with the 5.56 / .223 cartridge, especially so if using FMJ bullets that depend on fragmentation as a wounding mechanism such as the old SA military ball or US M193 / M855. 20 inch barrels will usually give around 3250 to 3300 FPS MV with M193...16 in cuts that by around 200 FPS to about 3050 or so, and 11.5in cuts it further to around 2900, and this ammo is full bore mil spec stuff. So it is easy to see how the Dashprod could fail to make power factor with 55gr ammo loaded to less than 'WFO' power (Wide Freaking Open). If you have a source you might be better off to try the 75-77gr bullets as they do not lose so much velocity when barrel length is reduced. Black Hills 75gr runs around 2700 in a 20in, 2650 or so in 16in and 2575 or thereabouts in a 14.5in. I could not find chrono figures for it in an 11.5 in or shorter barrel but extrapolation would suggest it should run at least 2400 and probably more...easily making the minimum power factor of 150.

For some reason I was thinking the little Dashprod had only a 8 or 9in barrel...can anyone clarify that? A 55gr at 2900 makes 159 and a 75gr at 2400 makes 180 by my calculations...but it IS early here and I am only on my second cup of coffee ;D

Dashprod has a 8.45 inch barrel apparently.

[ikor]

Well, a 55gr will have to do slightly more than 2700FPS and a 75gr 2000FPS or more to make the power floor...slightly more if one wants to be certain of making it under various conditions, etc. I would think that is do-able with the heavy bullet but I am not so sure about the 55gr pills.