When things don't compute

[Tnwhip]

My load, according to my chronograph with 162 A-max is 2900fps. That is fine and what it should be with my load but. With a 200 yd zero, The drop chart said to come up 7moa for 500 yards and that is low. Have to come up to 8.25 moa to hit center. To get a drop chart to works at 500, the fps has to be 2700 with a 8.25 moa. Why does it not compute right at 2900fps?

 

[SavageShtr]

That is the reason behind doing range varification!! The chrono graph is a good way to get an idea of the speed your bullet is traveling, but in most instances will not matchup in the field and will need to be tweaked up or down to match your drops. If one was to chrono there perticular load then just go hunting for instance you might be fine for the first couple hundred yards but the further out you went the further off target you would be and get very frustrated!! Now that you have your corrected velocity to match up with your 500 yard drops, go out and test it a little further out and see where you are at should be very close. Hope this helps!

 

[jrsolocam]

And you should check the elevation clicks to make sure they are what they are supposed to be.

 

[MMERSS]

Only trust your cronograph and click value so much until you have verified their accuracy. You're on the right track. Practice your drop chart or computed should hit to did hit data at various ranges and adjust your charts/programs to as closely as possible to where they match the actual drop.

 

[yobuck]

sorta like those gas milage stickers on car windows.
they lie also.
trust what you see not what youve been told.

 

[Wyofax]

Some of that deviation could be attributed to a variation in the bullets BC, could it not?

 

[azsugarbear]

Exactly my thought. What bullet are you shooting?

 

[D.ID]

I had this problem a while back except I kept hitting high. Ended up fudging my zero and scope hight, bc or adjustment size and all sorts of junk to try and make it fit. Ammo and time waisted, frustration and unreliable data.
Learned my lesson though, retired the chronograph and will never use it again.
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Check my scope adjustments for true value right off the bat as SOP during initial zeroing session.
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Do load development at long rang as a SOP now to test sd/es.
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Drop tests based on SWAG reverse engineered threw calculator or "trajectory validation" threw the G7 to identify velocity after accuracy has bean locked in is now my SOP.
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I personally believe that these procedures are the simplest, cheapest and most direct method of verifying everything and when it is done I already have the proof that it is dead on, because I based my calculations on reality with my individual system instead of basing my estimations on theory and still needing to go out and verify it.
Still using the modern ballistic calculator but simplifying the whole process and the equipment needed in order to get solid pre-verified dependable results.
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This is the better way in my opinion, Keeping it simple and based on fact.
Can I swear on pain of death my bullet is leaving at 2800 vs 2823: No.
Can I swear on pain of death that under (X) conditions, my elevation adjustment will be dead on using my scope with my rifle and load: ABSOLUTELY.

 

[bigbuck]

I had just the opposite problem as you have . MY Chrony said I was running 70 fps slower than what my trajectory Validate/field shooting said. So when I would dial the given MOA (from Exbol) I would hit high. In short I started with a chronied MV and ended up with Exbol having the final say. Now its dead nuts on the elevation dope. At 840 Yards I was 1'' high from dead center. I am getting 8 1/4 three shot groups at 1000 consistently . There is some guys on this site that can do much better so I am not bragging but this gives you an Ideah of wheather or not your gun is shooting close enough for everything to work right. It is possible your chrony is not accurate and your gun and ballistic software is doing what it is suppose too but like someone once told me (from this site) Garbage in Garbage out. If thats the case ? Good luck next time out........

 

[Tnwhip]

I am shooting a 280 Ackley. 26 1/2" barrel. I have worked up to this load that I got from someone else. 162A-max, 63.5g H-1000. I am getting 2915 avg. They were getting around 2940 fps. with 63.9g. So I was thinking my Velocity is about right. My new scope is a Vortex 6x24. What I have read about them is they track on the money. My clicks were dead on sighting it in at 200 yds. I will juggle things around and come up with a chart that works.
Having to use 2700 fps with a 200 yd zero in my drop chart seams like a lot when I am getting 2900 fps.

 

[cwinner]

You should definitely verify your clicks.....it amazes me that folks seem to believe that their scope tracks correctly just cause its brand X......

The first thing we do is verify our scopes, here's a picture of a recent test of my buddies night force NXS scope.


Rifle pictures:
https://www.icloud.com/photostream/#AD5oqs3qGn2l2J


This is from an extremely accurate 6.5 creedmoor rifle, reference the 5 shot group in the orange dot and two shots each at 0, 10, 20 & 30 MOA, notice their position to the lines we measured on the target.......

Is this a bad thing? Are we sending it back and crying about it? Absolutely not, all we do is enter a calibration factor for the scope clicks and continue with our field verification.

In this case using the MV calibration would indicate his rounds have a higher velocity that than they really do.........check your scope, use a chrony for a starting point and most importantly field verify every range you would take a shot at game from.

 

[azsugarbear]

There are a lot of variables that impact your drop chart. 200+ fps change to make your ballistics calc work right makes me think that something else is going on here.

Are you sure about scope height? What bullet are you shooting? (Depending on mfg., they can fudge their BC's a fair amount). Another way to throw off you calculations is to be off slightly off on your zero. If you are zeroed at 100 yds, that probably means you are hitting close to center - but not exactly on center. Let's say we are 1/8" below center at 100 yds. With our .25 MOA scopes, one click up puts our group 1/8" above center, while one click down puts us 3/8" below center - so we call it good - even though our group is 1/8" below center at 100 yds. That's as close as we can get it. So we enter a 100 yd zero into our ballistics. When validating, we find our bullet drop is 2-3 clicks below what it should be at 800 yds. That's because our true zero was not 100 yds, but closer to 80 yds.

My guess is you have several contributing factors that are combining together to get you off by that many clicks. Berger & Sierra bullets are pretty close to their stated BC's, but other mfgs bullets can be overstated. Just some additional thoughts to throw into the stew pot.