Range Report Need Help ... My dope failed ...

[Sapper524]

So while out shooting steel today I had one target that has me stumped. When shooting at this location we change firing points each trip to keep things interesting and keep the skills honed. Ranges can vary from 200 out to 800 depending on the firing point and elevation changes as well. The Rifle and load will shoot .5 MOA all day if I don't screw it up. Onto the meat of it ... Rifle Info

M77 Hawkeye 308 Win
20 inch 1:10 Heavy
SS 3-9 FFP / Badger Rings / Custom Rail (Ruger Options SUCK)

Load Data
175g SMK BTHP
RL15 44.2
OAL 2.855
CCI LR
Black Hills Brass
Velocity 2554 w/ SD=5 fps

Conditions
2394 Elevation
65 - 70 degrees
65% Humidity
30.05 Pressure

The load is slow but shoots GREAT out of this rifle and have great success with this. Ill Mil the target and estimate my range/ confirm with laser before I shoot. The shots when like this ...

633 yards
9 degree upslope
4.7 Mils Elevation dialed
HIT

471 yards
12 degree upslope
2.9 Mils Elevation
HIT

503 yards
10 degree upslope
3.3 Mils Elevation
MISS (called by spotter 14 inches high)
I scratch my head
-check scope mount
-check elevation adjustment
-check brass (primer and case look fine)
-reference my scope for adjustment off the spotters call
-dial down to 2.8 mils elevation
HIT - HIT - HIT

Wind was NOTTA and all ranges were confirmed with my ZIESS PRF Laser prior to sending anything. Using a protractor for angle reference. So at this point im stumped so out comes the chrony and ... 2552,2552,2551 are the results. Was not my chrony but mine gives almost exact numbers for this load. So when I get home I check my scope out, bag it, toss up my Mil Chart, elevation, windage, FFP all check out. This rifle and load have always been dependable and spot on. I didn't pull out a new box of reloads from my pack or truck with temp change. Im just stumped - any ideas ???

 

[Captain Kirk]

Re: Need Help ... My dope failed ...

What was your POI vs POA on the 471 target, high? After dialing 2.8 mils @ 503 yds, did you go back to 471 to see were you where at?


Kirk R

 

[Sapper524]

Re: Need Help ... My dope failed ...

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Captain Kirk</div><div class="ubbcode-body">What was your POI vs POA on the 471 target, high? After dialing 2.8 mils @ 503 yds, did you go back to 471 to see were you where at?


Kirk R </div></div>

No - one the 471 target POA/POI was good ... I did dial back up to 4.7 and Hit on the 633 again, after farting around with the Chrony. Your right though - checking at 471 would have been better. While on the Chrony I was shooting a tree stump at 100 (zero) and POA/POI was good.

 

[Skyking]

Re: Need Help ... My dope failed ...

Sometimes I have had the wrong BC. in my computer. And did not notice it got changed until I start shooting at longer ranges. Shoot at the 800 yard target and see what happens. But I guess you did shoot at the 633 target.

Also try a box test with your scope around the setting that you are see the changes. This may be a long shoot. But maybe something is wrong with the scope right in that area of adjustment?

 

[Grump]

Re: Need Help ... My dope failed ...

Ummm, without going into the actual numbers of how far off that *one* target's single hit was, compared to what you dialed for the next three hits, may I suggest...

The LRF gave you the distance to something behind the steel???

Occam's Razor leads me to that possibility. Further data might point elsewhere. Other things point *away* from the scope skipping movement on clicks, etc., (in the stripped thread/the thing broke theory), but there is also the equally plausible possibility of the erector tube/elevation function sticking and getting .5 mil off for just those shots, then un-sticking and returning to behaving itself afterwards.

The LRF mis-read could possibly be verified with a bit of Google Earth measurements if the aerial photos of that spot have sufficient landmarks, or by going back and "shooting" the distance from the midpoint and adding up the numbers. A friend of mine introduced me to the brilliant idea of leaving a target stand up at the firing line and shooting backwards to set the downrange targets. My critical thinking skillz and what Dad taught me about reducing variables and error lead me to suggest the midpoint ranging either way idea.

I suggest that this is worth checking out. Eliminating the LRF error, sadly, leaves you with the possibility of an internal scope malfunction that would be hard to replicate, if it even can be replicated. With mechanical devices that can fail (sound familiar???), I've personally experienced several provable single-event failures that to this day have not repeated themselves.

 

[Sapper524]

Re: Need Help ... My dope failed ...

Thanks Grump - great points. The spot where we shoot is in the mountains and the targets are placed on hillsides. Ranging is pretty easy with the laser. After this happened I probably checked the range with the laser 20 times and all were at 503 unless i went over the top of the target I got 505 or 506, its a pretty steep hill. The internal malfunction might be it. I am no scope internal guru so could dialing to max elevation then back to the bottom "reset" the erector. Then going back to my zero for chrony test, shooting the 633 from that zero. etc? I SHOULD have went back to the 503 target with the 3.3mil adjustment again. Hind Sight being what it is etc.

 

[Grump]

Re: Need Help ... My dope failed ...

Repeatability of measurement shows precision but not necessarily accuracy. It's a logic/instrumentation thing.

The setting points towards accuracy (what is behind is not very far behind), but only with regard to the two *most likely* points being measured. Other possible reasons for instrument error require bigger and bigger assumptions, thus less likely but certainly not impossible. For example, an error in the sensors or calculations software *might* lead to the LRF-equivalent of the stuck erector tube theory. Sadly, the voodoo and witchcraft of electronics can make those types of errors either *always* repeatable or *never* repeatable.

I would even wonder about an un-noticed bush in front...

 

[Grump]

Re: Need Help ... My dope failed ...

I just did a "reality check" with JBM using your reported velocity and conditions and got this:

625 -5.1 -114.7 1.5 32.7 1630.3 1.448 1032.6 0.918 2.2 48.5
650 -5.4 -127.2 1.5 35.7 1597.3 1.418 991.3 0.965 2.2 50.9

So, your first shot at 633 yards using 4.7 mils should have been *group centered* a tiny bit more than .4 mil low... add your rifles' precision level "shot probability cloud" around that and factor in your target size. 0.4 mil at 633 is about 9.16 inches...

IOW, JBM doesn't like your reported dope.

At that distance, my assumption of sight centerline of 1.7 inches above bore should not make much of a difference.

Checking 475 yards got me this:
475 -3.2 -55.5 1.0 17.8 1834.2 1.629 1307.0 0.658 2.0 34.7

3.2 mils, as compared to your 2.9 mils, should also have been *group centered* a tiny bit more than .4 mil low with your dope. At 475 (negligible difference to 471 from the LRF), .4 mil is a difference of 6.84 inches. How big was your steel? a 0.5-MOA group of about 2.4 inches, centered almost 7 inches low, would have hit where?

Now to 500 yards, JBM sez:

500 -3.5 -63.6 1.1 19.9 1799.5 1.598 1258.0 0.699 2.1 36.9

Only a 0.3 mil difference from 475 to 500...and your dope was 0.4 mil different...the same interval as the possible dope error if my calculations with JBM are correct. The .5 mil down you reported using equals a hair more than 9 inches at 503...

You got hits at 503 with 2.8 mils up, which is the OPPOSITE direction of the predicted errors from the other dope, possibly spot-on if the target were really closer to 425 than 450, thus way closer than 503 yards.

Just re-ran it with the 10 degree upslope, and at 500, it's still 3.5 mils up from a 100-yard zero.

Remember chaos theory--small errors in initial variations make for larger errors downstream in the calculations. I'd check some fundamentals, like where hits were centered, whether 100 yards was really 100 yards (it's gotta be way off to get .4 mil...that's 1.44 inches!), and re-verifying velocity.

14 inches high at 503 yards is a bit much, and the subsequent hits after sight correction point *away* from a random flyer, hitting a twig, shooter flinch, or other aberrations. The "shoulda been hitting .4 mil low on the other shots" factor makes this mystery even bigger, in magnitude of the POA/POI discrepancy.

14 inches high off the aim point, or 14 inches high from the edge of the target?

 

[Bigwheels]

Re: Need Help ... My dope failed ...

It could also be field positions with paralax error along with consistent/inconsistent cheek welds.

 

[Grump]

Re: Need Help ... My dope failed ...

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Bigwheels</div><div class="ubbcode-body">It could also be field positions with paralax error along with consistent/inconsistent cheek welds. </div></div>
Worth a check, too, FWIW. But the later shots show consistency for *those* shots.

IIRC, my parallax gives only about .2 mil optical error for 100- to further distances, and mirage can account for up to .3 mil. Combined for .5 and it's consistent with the other hits *if* all centered, so that leaves the dope as being for either a higher BC than Mr. Litz measured, a 100-y Z .4 mil high, a faster MV, or some combination of all three...as the most likely culprits.

A big difference in position for the 503-yard target, with less shoulder resistance and cheek pressure, by itself, for a .5-mil impact difference??? Or smaller and combined with the other three...?

 

[sobrbiker883]

Re: Need Help ... My dope failed ...

175SMK's going that slow do not equal "hit hit hit" at 2.8 mils for 503yds at 2300'asl usually. You 3.3 dope sounds closer.
I think you either experienced a wicked shift in DA or there is something else going on. (I'd agree you probably lased past the target).
Did you shoot after that and what was the results?

My 175's at that temp/elevation run 3 mils or so, and they pop out at 2695......

 

[Sapper524]

Re: Need Help ... My dope failed ...

As far as parralax error the scope was fixed parralax (SS 3-9) and I agree the the dope of 3.3 is what it should have been. Thats why im stumped. I was talking to my bud who was spotting at the time and he said my 633 yard hits were on the low side. Targets were 9 inches tall so that could mean up to 4 inches low or so. Im going to go shoot the box a couple times this weekend and see how it goes. If all goes well Im going to verify my ballistics again at know distance targets on level ground at 200/400/600.

 

[Sapper524]

Re: Need Help ... My dope failed ...

Well I shot a 8 wide 4 tall box today - went around 3 times. Everything checked out. It has to be a combination of small errors that lead up to what happened. Thanks for all the input.