.308 bullet speed variation

tmahawkcfi · October 27, 2008, 10:11 AM

I am loading .308 Sierra Matchking in 168 & 175 grns with 44.0 & 43.0 gns of Varget respectively with Lapua brass and CCI primers. Measuring with a Chrony we are getting 40 fps variations with the 168 gn and 60 fps variation with the 175 gn. Groups are fairly good averaging 3/4 moa @ 200 yds. What is considered a benchmark for variation in fps?

Zombie Steve · October 27, 2008, 05:22 PM

You are looking at extreme spread. A better indicator is the standard deviation of your data. Does your chrony give you that? I think it's safe to say if you can get your standard deviation to 10 - 15 fps, then you are really on top of your reloading game... a benchmark I'd say would be in the 15 - 20 fps range.

Good read on this subject here: http://www.naaminis.com/standev.html

Ultimately, if I was getting 3/4 moa at 200 yards, I wouldn't mess with it too much.

**Unclenick** · October 27, 2008, 09:47 PM

2% of velocity for extreme spread is pretty common. If you want to do better, you'll need to weigh every charge and use the mildest benchrest primer you can get your hands on. RWS primers are popular with bench rest shooters, when you can get them. I've also had good luck with Federal 210M.

Case uniforming is unnecessary with Lapua brass, but be sure they all have the same load history so any work hardening of the necks is matched to keep starting pressure the same. Let the gun cool off with equal timing between shots so you don't get temperature variation of the powder altering the burning speed. I know Varget claims its burning rate is temperature immune to 125°F, but YMMV. This article on the temperature effects is informative. The powder you expect to do best won't necessarily be best in your particular chambering. Besides, a chamber can get a good bit warmer than 125°F if you fire at more than a fairly slow pace. Letting a chambered round sit in the hot chamber will let it warm beyond that.

Count on building cartridges with the 175 grain SMK for long range. The 168 is a much older design that is typically unable to remain stable through the transonic velocity range. It starts tumbling and keyholing at about 700 yards when fired at .308 velocities. The 175 does not.

October 27, 2008, 10:11 AM	#1
tmahawkcfi Junior Member Join Date: November 7, 2006 Posts: 3	.308 bullet speed variation I am loading .308 Sierra Matchking in 168 & 175 grns with 44.0 & 43.0 gns of Varget respectively with Lapua brass and CCI primers. Measuring with a Chrony we are getting 40 fps variations with the 168 gn and 60 fps variation with the 175 gn. Groups are fairly good averaging 3/4 moa @ 200 yds. What is considered a benchmark for variation in fps?

October 27, 2008, 09:47 PM	#3
Unclenick Staff Join Date: March 4, 2005 Location: Ohio Posts: 21,061	2% of velocity for extreme spread is pretty common. If you want to do better, you'll need to weigh every charge and use the mildest benchrest primer you can get your hands on. RWS primers are popular with bench rest shooters, when you can get them. I've also had good luck with Federal 210M. Case uniforming is unnecessary with Lapua brass, but be sure they all have the same load history so any work hardening of the necks is matched to keep starting pressure the same. Let the gun cool off with equal timing between shots so you don't get temperature variation of the powder altering the burning speed. I know Varget claims its burning rate is temperature immune to 125°F, but YMMV. This article on the temperature effects is informative. The powder you expect to do best won't necessarily be best in your particular chambering. Besides, a chamber can get a good bit warmer than 125°F if you fire at more than a fairly slow pace. Letting a chambered round sit in the hot chamber will let it warm beyond that. Count on building cartridges with the 175 grain SMK for long range. The 168 is a much older design that is typically unable to remain stable through the transonic velocity range. It starts tumbling and keyholing at about 700 yards when fired at .308 velocities. The 175 does not. __________________ Gunsite Orange Hat Family Member CMP Certified GSM Master Instructor NRA Certified Rifle Instructor NRA Benefactor Member and Golden Eagle

October 27, 2008, 05:22 PM	#2
Zombie Steve Senior Member Join Date: June 3, 2007 Location: Old Colorado City Posts: 527	You are looking at extreme spread. A better indicator is the standard deviation of your data. Does your chrony give you that? I think it's safe to say if you can get your standard deviation to 10 - 15 fps, then you are really on top of your reloading game... a benchmark I'd say would be in the 15 - 20 fps range. Good read on this subject here: http://www.naaminis.com/standev.html Ultimately, if I was getting 3/4 moa at 200 yards, I wouldn't mess with it too much.