Do Test Groups' MOA Size Change With Range?

[stagpanther]

thanks!

[stagpanther]

I have a pressure trace and have seen some interesting behavior in pressure plots as the bullet goes down the bore. There's even a theoretical pulse that goes down the barrel and "rebounds" back to the projectile before it even exists the muzzle that I've heard about.

[Bart B.]

Originally Posted by stagpanther

I have a pressure trace and have seen some interesting behavior in pressure plots as the bullet goes down the bore. There's even a theoretical pulse that goes down the barrel and "rebounds" back to the projectile before it even exists the muzzle that I've heard about.

Yes, that shock wave goes back and forth in the barrel. When it is at the barrel muzzle, the claim is it enlarges the bore a few millionths inch and that degrades accuracy. Modes 4 and 8 in that link shows the muzzle diameter changing but no back and forth shock wave claimed in this web site:

http://www.the-long-family.com/OBT_paper.htm

I and others have had Garand match barrels whose bore at the muzzle enlarged well over a thousandth inch from steel cleaning rod wear that had no accuracy loss. They still tested sub MOA at 600 yards.

[stagpanther]

My tests have been on AR's--which are difficult because most of the chamber is usually covered by the barrel nut--and many of the barrels are made of SS416 which supposedly has an "erratic" molecular expansion and contraction behavior compared to say conventional 4150 CM.

[Unclenick]

I've done hoop stress analysis suggesting upwards of a quarter of a thousandth deformation can occur. The impulse seems to be initiated when the bullet hits the throat on its way out of the case. At that one moment, it pushes forward on the barrel and the reaction force applied to the breech end causes the chamber to stretch a little. As the bullet eases into the throat and gets to build up speed again, the chamber snaps back into position and those seem to be the pressure wave initiating forces.

Interestingly, those waves were first described (AFAIK) by Dr. Lloyd Brownell in his 1965 study of absolute pressure at the U of Michigan with funding from DuPont. You can seem them as small wiggles superimposed on the pressure curve where they occur. They don't seem to have equal magnitude in different loads and I have not done Pressure Trace readings to see if starting with the bullet in contact with the lands, which eliminates the bullet bumping into the throat, also mitigates the waves.

[Bart B.]

Unclenick, what are your thoughts on this Optimum Barrel Time theory:

http://www.the-long-family.com/OBT_paper.htm

[Cosine26]

Exterior Ballistics is a very complex subject. There is a very interesting, detailed and complex discussion and explanation of projectile stability in the July 1962 issue of the American Rifleman by E. H Harrison titled Projectile Stability. It gives a very detailed description of a projectile in flight and the forces acting on it.
In general, group size does not expand in a linear fashion with range because of changing forces acting on the projectile at various velocities and at various ranges.
The article just too long and too complex to copy and put on this forum.
The use of Doppler radar to measure inflight characteristics of projectiles available today gives much greater insight into exterior ballistics than existed in 1962.
FWIW

[Bart B.]

One can use one of the exterior ballistic software sites (JBM?) to see the bullet drop spread at different ranges with a 15 fps or 60 fps spread in muzzle velocity (or horizontal shot spread in a 1.5 to 6.0 mph spread in crosswind).

Change the bullet rated BC 1 to 2 percent lower to see cross wind drift changes as well as bullet drop variables.

[Unclenick]

Yes. Look at the table's total drop number for each range at each velocity. The difference in total drop for two velocities at any given range will be what you get for the vertical difference on target. You can also look at the sight elevation difference required to zero the two velocities at that range and convert that difference to drop at the desired range. Wind deflection distance differences also apply.


Bart,

I have run hoop stress analysis to determine what the waves might do to muzzle diameters and concluded that in some barrels they may contribute to the velocity flat spots you find shooting an incremental load string.

[Bart B.]

Originally Posted by Unclenick

I have run hoop stress analysis to determine what the waves might do to muzzle diameters and concluded that in some barrels they may contribute to the velocity flat spots you find shooting an incremental load string.

How much did the muzzle diameters change?