Acceptable extreme spread and standard deviation?

DaveInPA · January 17, 2009, 09:03 PM

When chronographing my .223 loads, what should I consider acceptable extreme spread and standard deviation in a 10 shot string?

PCJim · January 17, 2009, 11:24 PM

Dave, I go by the accuracy and size of the groupings compared in conjunction to the Chrony results. Usually, they go hand in hand. I think you'll find that the smaller the groupings, the lower the ES and SD of the rounds.

IllinoisCoyoteHunter · January 17, 2009, 11:29 PM

I read somewhere that benchrest shooters don't like to see anything more than 20-25 fps ES. I usually get 30 or 35 fps ES in 5 shot groups with my .243 with H4895. SD is usually around 14. Groups anywhere from .375" to .500" at 100 yds.

**Unclenick** · January 18, 2009, 12:16 AM

Charles Petty got 150 fps extreme spread from a load that proved accurate at 100 yards in his rifle. If you run range tables, you find that at around 3100 fps, a 150 fps difference causes the 55 grains Sierra BT to change vertical position about 0.25" at 100 yards. So you can still shoot a 0.25" group at 100 yards with that velocity difference. That is assuming the barrel time difference doesn't take your gun off a sweet spot.

When you get to 300 yards, a different story starts to develop, as the difference grows to almost 3" or not quite 1 moa. By 600 yards it is about 19" or 3 moa. You would normally choose a heavier, higher BC bullet for long range because it would be less sensitive to this problem, but the example is instructive. In general, if your purposes are short range, you don't really need to concern yourself much with velocity spread. As you get to longer ranges, you do.

January 17, 2009, 09:03 PM	#1
DaveInPA Senior Member Join Date: January 20, 2008 Location: Berks County, PA Posts: 1,106	Acceptable extreme spread and standard deviation? When chronographing my .223 loads, what should I consider acceptable extreme spread and standard deviation in a 10 shot string?

January 17, 2009, 11:29 PM	#3
IllinoisCoyoteHunter Senior Member Join Date: December 23, 2008 Location: Southern Illinois Posts: 1,527	I read somewhere that benchrest shooters don't like to see anything more than 20-25 fps ES. I usually get 30 or 35 fps ES in 5 shot groups with my .243 with H4895. SD is usually around 14. Groups anywhere from .375" to .500" at 100 yds. __________________ ~~IllinoisCoyoteHunter~~ ~NRA LIFE MEMBER~ ~NRA CERTIFIED INSTRUCTOR~

January 18, 2009, 12:16 AM	#4
Unclenick Staff Join Date: March 4, 2005 Location: Ohio Posts: 21,061	Charles Petty got 150 fps extreme spread from a load that proved accurate at 100 yards in his rifle. If you run range tables, you find that at around 3100 fps, a 150 fps difference causes the 55 grains Sierra BT to change vertical position about 0.25" at 100 yards. So you can still shoot a 0.25" group at 100 yards with that velocity difference. That is assuming the barrel time difference doesn't take your gun off a sweet spot. When you get to 300 yards, a different story starts to develop, as the difference grows to almost 3" or not quite 1 moa. By 600 yards it is about 19" or 3 moa. You would normally choose a heavier, higher BC bullet for long range because it would be less sensitive to this problem, but the example is instructive. In general, if your purposes are short range, you don't really need to concern yourself much with velocity spread. As you get to longer ranges, you do. __________________ Gunsite Orange Hat Family Member CMP Certified GSM Master Instructor NRA Certified Rifle Instructor NRA Benefactor Member and Golden Eagle

January 17, 2009, 11:24 PM	#2
PCJim Senior Member Join Date: July 11, 2008 Location: FL Posts: 570	Dave, I go by the accuracy and size of the groupings compared in conjunction to the Chrony results. Usually, they go hand in hand. I think you'll find that the smaller the groupings, the lower the ES and SD of the rounds.