Need trajectory help please

Wild Bill Bucks · November 12, 2009, 11:46 AM

I have a .308 Ruger with a competition barrel on it, that when sighted in dead on at 100 yards, is shooting exactly 2" low at 50 yards. All the software shows I should be about .5" low at 50 instead of 2".

Can anyone tell me why the rifle does this?

I'm shooting 1/4" groups at 100 yards and I don't want to change anything, but I sure would like to know what my trajectory should be out to 250 yards or so.

Statistics are: 2822 fps (Chrony) Bullet is 150 grain( Moly coated ballistic tip, boat tail, from combined Technologies) B.C. is .442, and the scope is sitting exactly 1.5" above the barrel center.

Has anyone got any software that can give me some trajectory numbers for this load?

Thanks in advance.

Scorch · November 12, 2009, 12:03 PM

Wild Bill-
Let me see if I got this right:
Your rifle is sighted to point-of-aim (POA) at 100 yds, having risen 1.5" from the bore line to line-of-sight (LOS). Assuming this is true, at 50 yds, POI should be roughly 1/2"-3/4" below LOS. But at 50 yds, it drops below the line-of-bore to the target, then rises again out to 100 yds? I have a hard time with the line of flight on that one. Bullets do not deviate in flight, they travel from bore to target in an arc, albeit a fairly flat arc, that can be approximated to be a straight line (sometimes a tight spiral, but still fairly straight).

I have seen bullets shoot tighter at 200 yds than at 100 yds due to very tight barrel twist and very light bullets, but I have no way of knowing if the deviation was as large as you are seeing, or if this could account for your observed deviation. Perhaps you could shoot at incremental distances out to 100 yds and get an approximation of your trajectory.

Wild Bill Bucks · November 12, 2009, 12:27 PM

Scorch,

This makes no sense to me either, that's why I posted. I measured the shots with a ruler, and my shots are exactly 2" low at 50 and dead on at 100.

I measured the scope height from the center of the barrel to the center of the scope lense with a ruler. Could I be doing the measurements wrong?
The range I'm at doesn't allow for any other distances but 50 and 100 so I can't tell what the bullet is doing at any other ranges. If my scope measuring is wrong, and the scope is actually 2" high then I could see the POI being that low, but like you, I can't figure out how a bullet falls below LOS further than the height of the scope.

I'm at work right now but I will re-measure the scope again when I get home, just to be real sure.

saudst · November 12, 2009, 06:37 PM

Hey Wild Bill,

Try this site. It's the one I've been useing for years and it's free. I have saved it to my favorites in my loading section and love it. Just pay attention to the info you put in, I zero some of it out as I don't use most of it, just the trajectory, wind drift and retained energy. The cards are great, I laminate them for my stock.

Tim.

http://www.jbmballistics.com/calcula...ulations.shtml

http://www.jbmballistics.com/cgi-bin/jbmtraj-5.0.cgi

**Unclenick** · November 12, 2009, 06:57 PM

Because the bullet starts out at -1.5", and must head upward to make the zero at 100", the result is physically impossible for the gun in a machine rest. That leads me to conclude it's a sight problem. If the scope has fixed parallax, then the parallax is likely fixed at 100 yards and will be out at 50 yards making perfect head and eye alignment with the optics essential to avoid a change in POI. If the scope has a parallax adjustment and you are using it, and/or the focus adjustment, that suggests the scope is being thrown off by altering these adjustments, and needs service.

Nick

Hog Buster · November 12, 2009, 08:05 PM

"perfect head and eye alignment with the optics essential"

Unk you need to emphisize this a bit more often. Lots of shooters think all you need to do is put the crosshairs on the bullseye.

Wild Bill Bucks · November 13, 2009, 08:53 AM

Unc,

Perfect answer. That is exactly what my problem is. I had my AO adjusted to the 100 yard mark, and had not considered re-adjusting it to 50 yard, when I moved to the 50 yard range. Makes complete sense. Thanks a bunch.

GeauxTide · November 13, 2009, 09:00 AM

A vast storehouse of knowledge.

November 12, 2009, 11:46 AM	#1
Wild Bill Bucks Senior Member Join Date: December 28, 2005 Location: Southeastern Oklahoma, Next door to Sasquatch Posts: 1,266	Need trajectory help please I have a .308 Ruger with a competition barrel on it, that when sighted in dead on at 100 yards, is shooting exactly 2" low at 50 yards. All the software shows I should be about .5" low at 50 instead of 2". Can anyone tell me why the rifle does this? I'm shooting 1/4" groups at 100 yards and I don't want to change anything, but I sure would like to know what my trajectory should be out to 250 yards or so. Statistics are: 2822 fps (Chrony) Bullet is 150 grain( Moly coated ballistic tip, boat tail, from combined Technologies) B.C. is .442, and the scope is sitting exactly 1.5" above the barrel center. Has anyone got any software that can give me some trajectory numbers for this load? Thanks in advance.

November 12, 2009, 12:03 PM	#2
Scorch Senior Member Join Date: February 13, 2006 Location: Washington state Posts: 15,248	Wild Bill- Let me see if I got this right: Your rifle is sighted to point-of-aim (POA) at 100 yds, having risen 1.5" from the bore line to line-of-sight (LOS). Assuming this is true, at 50 yds, POI should be roughly 1/2"-3/4" below LOS. But at 50 yds, it drops below the line-of-bore to the target, then rises again out to 100 yds? I have a hard time with the line of flight on that one. Bullets do not deviate in flight, they travel from bore to target in an arc, albeit a fairly flat arc, that can be approximated to be a straight line (sometimes a tight spiral, but still fairly straight). I have seen bullets shoot tighter at 200 yds than at 100 yds due to very tight barrel twist and very light bullets, but I have no way of knowing if the deviation was as large as you are seeing, or if this could account for your observed deviation. Perhaps you could shoot at incremental distances out to 100 yds and get an approximation of your trajectory. __________________ Never try to educate someone who resists knowledge at all costs. But what do I know? Summit Arms Services

November 12, 2009, 06:37 PM	#4
saudst Senior Member Join Date: February 20, 2008 Location: Central Wisconsin Posts: 184	Hey Wild Bill, Try this site. It's the one I've been useing for years and it's free. I have saved it to my favorites in my loading section and love it. Just pay attention to the info you put in, I zero some of it out as I don't use most of it, just the trajectory, wind drift and retained energy. The cards are great, I laminate them for my stock. Tim. http://www.jbmballistics.com/calcula...ulations.shtml http://www.jbmballistics.com/cgi-bin/jbmtraj-5.0.cgi __________________ Tim. "Fear of death will not prevent dieing, but it may prevent living". Last edited by saudst; November 12, 2009 at 06:48 PM. Reason: More info.

November 12, 2009, 06:57 PM	#5
Unclenick Staff Join Date: March 4, 2005 Location: Ohio Posts: 21,057	Because the bullet starts out at -1.5", and must head upward to make the zero at 100", the result is physically impossible for the gun in a machine rest. That leads me to conclude it's a sight problem. If the scope has fixed parallax, then the parallax is likely fixed at 100 yards and will be out at 50 yards making perfect head and eye alignment with the optics essential to avoid a change in POI. If the scope has a parallax adjustment and you are using it, and/or the focus adjustment, that suggests the scope is being thrown off by altering these adjustments, and needs service. Nick __________________ Gunsite Orange Hat Family Member CMP Certified GSM Master Instructor NRA Certified Rifle Instructor NRA Benefactor Member and Golden Eagle

November 12, 2009, 08:05 PM	#6
Hog Buster Senior Member Join Date: October 26, 2009 Location: Pointe Coupee, Louisana Posts: 772	"perfect head and eye alignment with the optics essential" Unk you need to emphisize this a bit more often. Lots of shooters think all you need to do is put the crosshairs on the bullseye. __________________ Those who beat their guns into plows, will plow for those who don't.-Thomas Jefferson

November 12, 2009, 12:27 PM	#3
Wild Bill Bucks Senior Member Join Date: December 28, 2005 Location: Southeastern Oklahoma, Next door to Sasquatch Posts: 1,266	Scorch, This makes no sense to me either, that's why I posted. I measured the shots with a ruler, and my shots are exactly 2" low at 50 and dead on at 100. I measured the scope height from the center of the barrel to the center of the scope lense with a ruler. Could I be doing the measurements wrong? The range I'm at doesn't allow for any other distances but 50 and 100 so I can't tell what the bullet is doing at any other ranges. If my scope measuring is wrong, and the scope is actually 2" high then I could see the POI being that low, but like you, I can't figure out how a bullet falls below LOS further than the height of the scope. I'm at work right now but I will re-measure the scope again when I get home, just to be real sure.

November 13, 2009, 08:53 AM	#7
Wild Bill Bucks Senior Member Join Date: December 28, 2005 Location: Southeastern Oklahoma, Next door to Sasquatch Posts: 1,266	Unc, Perfect answer. That is exactly what my problem is. I had my AO adjusted to the 100 yard mark, and had not considered re-adjusting it to 50 yard, when I moved to the 50 yard range. Makes complete sense. Thanks a bunch.

November 13, 2009, 09:00 AM	#8
GeauxTide Senior Member Join Date: April 20, 2009 Location: Helena, AL Posts: 4,423	Unclenick is the man A vast storehouse of knowledge.

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