Data not matching shooting

So I finally had a chance to take a poke at the 500 yd steel at the Cheyenne Mountain Shooting Complex yesterday.

I've used both JBM ballistics calculator and the excel spreadsheet provided by meccastreisand on YouTube as well as the spreadsheets
by Tiborasorus Rex. All three data sources have indicated that with my load, I had 2.6 Mils of drop at 500 yards. The actual shooting
proved that I needed closer to 5 Mils of adjustment. I've double checked my inputs for the programs and all seems to be good.

Any ideas on why my actual drops are so far off from my data?

s

Lots of variables here. First, have you chronographed your load? Guessing or assuming a loads velocity off published data can throw things off.

That's is where I would start as well. Chrono the round you are using to verify velocity.

You dont need a chrono.

Get a rock solid zero at 100.

Then figure out how much drop you are getting at 300. Select your bullet in a ballistic program and do the match backwards to find out how fast the bullet is going. You might need to tweak it some more out past 800 or so. It will get you within a couple inches out past 600.

What load?

If 2.6 is really right for 500, 5.0 should get you close to 800.

Yep, have chrono'd this particular load with an average velocity of 3296 fps, averaged over more than 100 measured rounds. ES over that 100 rounds is just under 32 FPS as these are all hand trickled loads.
And while I know that this load is not really appropriate for longer range shooting, it's what I had handy and what I worked up the data for before I went to the range yesterday. The load is 40 Gr VMax over 25.7
gr IMR 8208 XBR.

I know something has to be out of whack with my inputs since the 5 mil drop was getting me hits every time on the 500 yd steel.

S

A quick run through my app gave 3.0 mils to 500. Something is definitely goofy. 100 yard zero?

Yep, 100 yd zero. Maybe time to get that scope in your jig? It has recovered from the trauma sustained last year.

Sam

What is the height over bore of the centerline of the optic?

3 inches. Scope is a Falcon Menace 4-14 X 44. It's a little higher than I'd like it but it's the best I can do right now.

s

My app says 3.6 mils to 500.

what app are you using?

JBM Ballistics: http://www.jbmballistics.com/cgi-bin/jbmtraj-5.1.cgi Which gives 2.6 Mils
Shooter Iphone App which gives 2.8 mils
and the Spreadsheet provided by meccastreisand which can be downloaded at http://www.youtube.com/watch?v=RR-1WIvvH1E. This also gives 2.6 mils.

s

Parallax.

I've learned (a pretty expensive and hard way) that getting my parallax wrong at 100 yards can and will multiply out to distances, and will throw your whole DOPE off.

That's just me though...

Hmmmmm. Hadn't thought about parallax. I thought I had it pretty well dialed in but I think I'll put more thought into it next time I go.

I started with getting a solid 100 yard zero, and remembering everything I did behind the rifle to make those shots...

and parallax use to kick my ass cause I do occasionally get a little complacent.

Your cold bore shot/first shot of the day will let you know if your doing something wrong with the idea that your zero doesn't change.
Figuring that out... then you move out further.

I'm only out to 400 or 500 yards for solid DOPE right now... anything further... I'm SWAG'ng it [Coffee]

Parallax is a good guess. It would have to far enough off that the image would be pretty fuzzy, but it's very possible. I don't suppose the parallax knob has gone untouched since you shot, has it?

If not parallax, I'm guessing a broken scope.

Were you dialing in the correction or holding over?? If holding over, was your scope set to the calibrated magnification? Shooting into the wind? 40gr pills fall pretty quick after 300yds and a little wind could make it worse.

No, I adjusted the parallax to get the target as in focus as I could detect with my old eyes. I was dialing in the correction and don't (shouldn't) need to set scope magnification since it's an FFP scope. Wind was about 10 mph at my 300 degree position...so, a quartering headwind. I originally dialed in the 2.6 mils of drop and observed from the hits in the dirt that I was quite a bit low. Then I walked it up using my Mil reticle. Some of the other guys who were watching said they were able to observe the hits in the dust as I walked it up and then stopped seeing hits, indicating that I was probably hitting the steel (lots of shooters so I couldn't hear the hits).

s

Probably not parallax then. You should have been close enough. I think your scope is at fault. You should need somewhere between 2.5 mils and 3.0 mils to get to 500. Needing double that tells me the reticle is not moving how you think it should be.

Were you shooting prone or off a bench? Bipod or bags? I don't know why I ask, just looking for a clue.

Not meaning to be offensive, but are you an accomplished shooter? I've seen guys at PD matches miss targets by many feet at 300-400 yards due to a lack of experience or skill.

Just thought of something else. Could be vertical wind. Were these shots in rapid succession or spread out over time? Have you repeated the experiment?

Certainly no offense taken Tim. And no, I'm not an accomplished shooter by anyone's estimation. I was shooting off a bipod in the front, bag in the rear from a concrete bench at the time. I'm pretty sure I wasn't slapping the trigger or anything like that. With this rifle I'm capable of pretty consistently putting 5 rounds in 0.6 inches at 100 yards. I did shoot a box test on the scope earlier in the day and got fairly satisfactory results considering it was the first shooting I had done for the session and that's usually the worst part of my day.

Sam

Well, the question arises as to the reliability of the BC value for the bullet.

Over a Mil difference between calculated and observed is huge . For the calculated numbers to be off that much the inputs to the program would need to be totally screwed . Box test the scope in a large increment , like 10 Mils plus , small 1 or 2 Mil box's aren't big enough to show a trend at close range . Those scopes don't have the best reputation for reliability .

Run it up here and we will put it in the test jig.