What am I missing? Different brass/bullets, group moves left?

Kjeil · January 16, 2009, 09:21 PM

I finally made it out to the range today with a working scope to try different loads and I saw something that I just don't get.

When I switched to loads using different brass and bullets the group's center moved down and to the left about two inches.

I can understand the drop as the second group probably had more capacity and therefore lower pressure and it had a bigger bullet therefore it was slower. But why would it move to the left?

BTW, this wasn't just a handful of rounds. It was two groups of 40 (10 rounds of four different loads). Here are the details:

Rifle specs:
5.56mm (Military) Chambered AR
16" Barrel
Chrome Lined chamber and bore
Mid-Length gas system
1:7 Twist

Batch 1 (four batches of ten)
Brass: WCC
Primer: CCI Arsenal
Bullet: Sierra Matchking 69gr
C.O.L.: 2.26
Powder: Varget
Charge: 23,23.5,24,24.5

Batch 2 (four batches of ten)
Brass: Hordany Match
Primer: CCI Arsenal
Bullet: Hornady BTHP 75gr
C.O.L.: 2.25
Powder: Varget
Charge: 19.5,20.0,20.5,21

Weather:
40 degrees
No Wind

The one possibility I can think of is that the Hornady brass was brand new and I didn't realize you still had to size it (You may remember the "Oops" thread) But I still don't see why that would move it to the left.

Any ideas?

IllinoisCoyoteHunter · January 16, 2009, 10:45 PM

when you fire a rifle, vibrations occur in the barrel. the vibration runs to the tip of the barrel and back (and forth). With various charges, the point of impact can change if the "vibration" happens to be at the tip of the barrel as the bullet is exiting the muzzle. To make it easier to unserstand, think of it as the barell having a wave running thru it. If the wave and bullet get to the muzzle at the same time, the wave can "flip" the bullet off target. SO, you need to find the "sweet spot" powder charge where the "wave" is not at the muzzle when the bullet exits. OR, you may just be a bad shot!!!! Just kiddin'

IllinoisCoyoteHunter · January 16, 2009, 10:47 PM

could be a combination of this and cant error...

Scorch · January 17, 2009, 12:13 AM

Harmonics. Simple answer is what Illinois Coyote Hunter said. The complicated answer takes time, but essentially it's how the barrel vibrates when the bullet travels down it. Change powder, primers, bullet, anything at all, and it vibrates differently.

**Unclenick** · January 17, 2009, 12:49 AM

The guys have it right. More specifically, it will be due to the differences in barrel times for the two loads letting the bullets exit at different phases of the barrel's harmonic whipping motion. Usually that motion is mostly up and down, but if your gun has a lateral recoil moment, you will get some harmonic whipping on that axis as well. Floating the barrel tends to limit lateral whipping, but it can happen just due to how the gun recoils into your shoulder.

January 16, 2009, 09:21 PM	#1
Kjeil Member Join Date: December 29, 2008 Posts: 32	What am I missing? Different brass/bullets, group moves left? I finally made it out to the range today with a working scope to try different loads and I saw something that I just don't get. When I switched to loads using different brass and bullets the group's center moved down and to the left about two inches. I can understand the drop as the second group probably had more capacity and therefore lower pressure and it had a bigger bullet therefore it was slower. But why would it move to the left? BTW, this wasn't just a handful of rounds. It was two groups of 40 (10 rounds of four different loads). Here are the details: Rifle specs: 5.56mm (Military) Chambered AR 16" Barrel Chrome Lined chamber and bore Mid-Length gas system 1:7 Twist Batch 1 (four batches of ten) Brass: WCC Primer: CCI Arsenal Bullet: Sierra Matchking 69gr C.O.L.: 2.26 Powder: Varget Charge: 23,23.5,24,24.5 Batch 2 (four batches of ten) Brass: Hordany Match Primer: CCI Arsenal Bullet: Hornady BTHP 75gr C.O.L.: 2.25 Powder: Varget Charge: 19.5,20.0,20.5,21 Weather: 40 degrees No Wind The one possibility I can think of is that the Hornady brass was brand new and I didn't realize you still had to size it (You may remember the "Oops" thread) But I still don't see why that would move it to the left. Any ideas? __________________ Since everbody asks: "Kjeil" is pronounced "Kyle" "If thou hast not a sword, sell thy cloak and buy one" Luke 22:36

January 16, 2009, 10:45 PM	#2
IllinoisCoyoteHunter Senior Member Join Date: December 23, 2008 Location: Southern Illinois Posts: 1,527	when you fire a rifle, vibrations occur in the barrel. the vibration runs to the tip of the barrel and back (and forth). With various charges, the point of impact can change if the "vibration" happens to be at the tip of the barrel as the bullet is exiting the muzzle. To make it easier to unserstand, think of it as the barell having a wave running thru it. If the wave and bullet get to the muzzle at the same time, the wave can "flip" the bullet off target. SO, you need to find the "sweet spot" powder charge where the "wave" is not at the muzzle when the bullet exits. OR, you may just be a bad shot!!!! Just kiddin' __________________ ~~IllinoisCoyoteHunter~~ ~NRA LIFE MEMBER~ ~NRA CERTIFIED INSTRUCTOR~

January 16, 2009, 10:47 PM	#3
IllinoisCoyoteHunter Senior Member Join Date: December 23, 2008 Location: Southern Illinois Posts: 1,527	could be a combination of this and cant error... __________________ ~~IllinoisCoyoteHunter~~ ~NRA LIFE MEMBER~ ~NRA CERTIFIED INSTRUCTOR~

January 17, 2009, 12:13 AM	#4
Scorch Senior Member Join Date: February 13, 2006 Location: Washington state Posts: 15,248	Harmonics. Simple answer is what Illinois Coyote Hunter said. The complicated answer takes time, but essentially it's how the barrel vibrates when the bullet travels down it. Change powder, primers, bullet, anything at all, and it vibrates differently. __________________ Never try to educate someone who resists knowledge at all costs. But what do I know? Summit Arms Services

January 17, 2009, 12:49 AM	#5
Unclenick Staff Join Date: March 4, 2005 Location: Ohio Posts: 21,063	The guys have it right. More specifically, it will be due to the differences in barrel times for the two loads letting the bullets exit at different phases of the barrel's harmonic whipping motion. Usually that motion is mostly up and down, but if your gun has a lateral recoil moment, you will get some harmonic whipping on that axis as well. Floating the barrel tends to limit lateral whipping, but it can happen just due to how the gun recoils into your shoulder. __________________ Gunsite Orange Hat Family Member CMP Certified GSM Master Instructor NRA Certified Rifle Instructor NRA Benefactor Member and Golden Eagle