runout/concentricity question

bow shot · September 9, 2009, 11:59 AM

I understand that it does matter, but I don't understand why. I mean, so the bullet starts into the barrel cocked...doesn't it have enought to make it fly straight out of the barrel, ie., being forced at a jillion psi down the barrel, that chrushes it tight in the bore it travels down 20+ inches???

Help me understand...

**Unclenick** · September 9, 2009, 12:08 PM

The bullet metal is soft enough for the rifling to engrave it. Therefore it is soft enough that it can be forced into the bore at a slight angle. We're talking less than a quarter of a degree for most jacketed bullets. Not much. But it is enough that the engraving of the rifling leaves the bullet slightly out of balance, and the base of the bullet doesn't leave the muzzle perfectly square, so the bullet nose goes through extra coning motion it has to recover from. These things don't happen perfectly uniformly from one shot to the next, so the scatter of the bullets POIs on the target is increased.

brickeyee · September 9, 2009, 12:08 PM

It can engrave into the rifling slightly cocked and just continue from there.

As soon as it leaves the barrel aerodynamic and gyroscopic forces govern its flight.

Even small variations can affect accuracy, especially if the gun is accurate to start with.

wncchester · September 9, 2009, 03:16 PM

" I mean, so the bullet starts into the barrel cocked...doesn't it have enought to make it fly straight out of the barrel, ie., being forced at a jillion psi down the barrel, that chrushes it tight in the bore it travels down 20+ inches???"

It does seem that way, don't it? But it has been well proven to not be true. To shoot straight it has to start straight.

bow shot · September 10, 2009, 11:15 AM

Understood. Thanks for the straight answers folks.

September 9, 2009, 11:59 AM	#1
bow shot Senior Member Join Date: March 26, 2009 Posts: 263	runout/concentricity question I understand that it does matter, but I don't understand why. I mean, so the bullet starts into the barrel cocked...doesn't it have enought to make it fly straight out of the barrel, ie., being forced at a jillion psi down the barrel, that chrushes it tight in the bore it travels down 20+ inches??? Help me understand...

September 9, 2009, 12:08 PM	#2
Unclenick Staff Join Date: March 4, 2005 Location: Ohio Posts: 21,063	The bullet metal is soft enough for the rifling to engrave it. Therefore it is soft enough that it can be forced into the bore at a slight angle. We're talking less than a quarter of a degree for most jacketed bullets. Not much. But it is enough that the engraving of the rifling leaves the bullet slightly out of balance, and the base of the bullet doesn't leave the muzzle perfectly square, so the bullet nose goes through extra coning motion it has to recover from. These things don't happen perfectly uniformly from one shot to the next, so the scatter of the bullets POIs on the target is increased. __________________ Gunsite Orange Hat Family Member CMP Certified GSM Master Instructor NRA Certified Rifle Instructor NRA Benefactor Member and Golden Eagle Last edited by Unclenick; September 9, 2009 at 12:22 PM.

September 10, 2009, 11:15 AM	#5
bow shot Senior Member Join Date: March 26, 2009 Posts: 263	got it. Understood. Thanks for the straight answers folks.

September 9, 2009, 12:08 PM	#3
brickeyee Senior Member Join Date: December 29, 2004 Posts: 3,351	It can engrave into the rifling slightly cocked and just continue from there. As soon as it leaves the barrel aerodynamic and gyroscopic forces govern its flight. Even small variations can affect accuracy, especially if the gun is accurate to start with.

September 9, 2009, 03:16 PM	#4
wncchester Senior Member Join Date: December 1, 2002 Posts: 2,832	" I mean, so the bullet starts into the barrel cocked...doesn't it have enought to make it fly straight out of the barrel, ie., being forced at a jillion psi down the barrel, that chrushes it tight in the bore it travels down 20+ inches???" It does seem that way, don't it? But it has been well proven to not be true. To shoot straight it has to start straight.