Need trajectory help please

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Wild Bill Bucks

New member
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.
 
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Scorch

New member
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). :confused:

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.
 
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Wild Bill Bucks

New member
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.
 
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saudst

New member
Last edited:
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Unclenick

Staff
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
 
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Hog Buster

New member
"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.
 
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Wild Bill Bucks

New member
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.
 
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