Verifying Drop

[Ucsdryder]

With all the ballistic calculators out now, the only real wild card seems to be B.C. BC isn't something easily measured and it seems manufacturers have a tendency to exaggerate the number. Speed, twist, environment, bullet weight, etc are all pretty simple to confirm. It seems like if you could get your BC correct on a ballistic calculator and verify drop at whatever range you choose, all the other ranges would fall into place.

Let me try to explain better. If your ballistic calculator states at 1000 yards your drop should be 7.8 mils but your true drop is 8.1 mils, if you play with the BC on your calculator until your calculator gives you 8.1 mils at 1000, all the other distances should fall into place. A spot check here and there and it would be a pretty easy way of verifying drop. Has anybody tried this method?

The other method would be to shoot a 3-5 shot group from 100 to your maximum shooting distance. It wouldn't use a ton of ammo, let's say 30-50 rounds, but moving a target in 100 yard increments every 5 shots could take the better part of a day, would be decently expensive, and a lot of guys can't put 50 rounds down range with a hunting rifle due to recoil.

 

[Taylorbok]

I shoot groups and verify drop at 300 yards. My thought on that is the conditions will have almost no affect on it at that distance (I stole this from David Tubb). Out at 1000 you can see a pretty significant change in wind direction and speed that can contribute to a fairly large change in POI. You also have to consider that your chrono will have a margin of error around .25% which isn't lots but thats 7.5 fps on a 3000 fps bullet, it all adds up when you start getting out there.

JBM has a calculator that you can use to verify using near and far velocity. what I have done is get a near velocity then take my chrono out to 100 and shoot through it. then take it out to 300, then I sit down and plug the numbers in and then cross them to what I found for my drop and the advertised then see how close they all are. My calculated were very close and I decided to just average them.

Strelok Pro offers a Multi BC calculation, my plan is once my barrel is broken in to use JBM to calculate BC off of a velocity at 500 and 1000 yards to use this function

 

[338 dude]

With all the ballistic calculators out now, the only real wild card seems to be B.C. BC isn't something easily measured and it seems manufacturers have a tendency to exaggerate the number. Speed, twist, environment, bullet weight, etc are all pretty simple to confirm. It seems like if you could get your BC correct on a ballistic calculator and verify drop at whatever range you choose, all the other ranges would fall into place.

Let me try to explain better. If your ballistic calculator states at 1000 yards your drop should be 7.8 mils but your true drop is 8.1 mils, if you play with the BC on your calculator until your calculator gives you 8.1 mils at 1000, all the other distances should fall into place. A spot check here and there and it would be a pretty easy way of verifying drop. Has anybody tried this method?

The other method would be to shoot a 3-5 shot group from 100 to your maximum shooting distance. It wouldn't use a ton of ammo, let's say 30-50 rounds, but moving a target in 100 yard increments every 5 shots could take the better part of a day, would be decently expensive, and a lot of guys can't put 50 rounds down range with a hunting rifle due to recoil.

Yes I have played with BC to get my data to line up ,another thing is run multiple bc's you can purchase a custom curve through certain ballistic solver's or if you refer to Bryan Litz's book ballistic performance of rifle bullets it gives you his multiple bc's

from different velocities that he has recorded through real world testing

 

[bozoben]

I'm fairly new at this game and this is what I just got done doing (yesterday actually).

First things first obviously and made sure my scope/mount/rings were tight and torqued.

Then confirmed a dead on zero with 3 rounds in 3/4" @ 200 yards.

Then I put all my data into a ballistics calculator to get me a preliminary drop chart that would get me on paper. Planned to take groups @ 600 and change input data to match my true drops to get me on paper @ 1000.

Set up a target @ 600 and shot a group. Group happened to be 5.3" centered vertically but 3 inches left. Not worrying about windage at this point, just drop so I was happy. Didn't make any adjustments to the chart. At that point if I needed to adjust drops I planned to plug different velocities into my calculator to match my true drops.

Put up a target @ 1000 and shot my last 3 rounds. 3 rounds in a 7 inch group centered about 5 inches left and 5 inches high of bulls eye. At this point I could make some very minor adjustments to my chart to get dead on @ 1000 but to be perfectly honest I was tickled to death with it just the way it was.

I think I'll send a few more rounds down range on different days before I adjust anything.

Been working on this rifle for quite some time now and I'm very pleased to be able to shoot it under 1 MOA out to a grand.

I think this is more or less what you are talking about doing. Worked for me.

 

[Jeff Heeg osoh]

Just a couple things that will affect end results

1st the scope.
we will setup at a measured 100 yards and hang a 72" carpenters rule that is plumb vertical. Then we will lock a block down in a vice that has a picatinny rail mounted to it and a scope we want to check mounted to the rail. I still used a comparator and marked the Mils on the rule as well as check the inches used for a moa scope, its amazing that some carpenter rules are not as accurate as you would think. Then we will crank the turrets and check the actual travel of the cross hairs - retical and record the measurements in 10 mil increments or the actual travel in moa versus what was dialed. The tooling in the threads on the scope turretts is what causes a difference and we have seen this in about any high end scope manufactured. This seems faster, cheaper and more accurate then shooting rounds off at a box target but most importantly it eliminates any error from accuracy in the shooter and or ammo. Most Balistics programs have a scope correction factor that you can now enter in what you found out after doing this test. Note it is also nice to see if your retical tracks straight with the plumbed carpenters rule when dialing it up and down.

BC and manufactures ratings
Two things affect what a honest manufacturer rated BC is versus yours.
The fps speed they were tested at the manufacture facility versus yours will change BC values.
Most importantly the groves in the barrel will distort a bullet differently from one barrel to the next when fired and the end result is the grooves in the bullet now can be more distorted and will effect the BC efficiency of the bullet versus if it was shot from another rifle.

The DA will also throw a wrench in the gears as far as your impacts versus what was posted on a label or from one day to the next or morning versus hot afternoons after you think you have everything figured out.

For us once we have calibrated the scope travel in the retical versus what was dialed then entered that in the scope correction values on the program we will shoot groups at 800 or 1000 and tweak the BC value in the program to make the new info on what to dial make sense with the impacts. For some folks this may mean shooting groups at 300 or 500 - wherever they can get good feedback on a calm overcast day that is within their skill level with good feedback. The overcast and calm days do help with wind issues pushing a bullet and less mirage distorting the target.

Hope this helped some.
JH

 

[Taylorbok]

You can check scope tracking and level at the same time with a tall target test. I do it at 50 yards to check full scope travel. (110 moa is to tall to test at 100) 4x8 ply wood. Draw a plum line with an aim mark at the bottom. I draw a mark every full revolution of the turret, in my case 25 moa, (every 13" at 50 yards) shoot at the aim point and keep dialling up . If the bullet lands on the mark its all good, if it's low or high you must add a correction factor. If it starts to veer left or right scope isn't perfectly level

 

[BallisticsGuy]

What you're talking about is BC truing and it's a vital part of practical external ballistics. Any given BC is only accurate at a particular velocity and there are other issues around BC that complicate things, like is the projectile compliant to the form factor of the model being used. A G7 number given for a bullet that's not G7 compliant isn't going to produce good results anyway. Some companies like Sierra give their BC's in speed regimes because they recognize that a BC given for 2600fps is going to be different to one given for 1600fps. When you have a calculator that accepts multiple BC/velocity bands you get the benefit of Sierra publishing that kind of data. For companies that don't publish that data, they are providing a single BC based on how fast the bullet will be pushed initially and how far they think you'll be shooting and kind of peg it to that. So if the bullet is a 150gr .308" hunting bullet you'll probably be given a BC that's tuned inside of 400yrds at moderate velocity. A long range target bullet that's heavy for caliber like a 115gr .243" VLD would probably be pegged to a longer distance, like 800yrds and an anticipated MV that's a good bit higher.

The way I do it is to start at 500m and work forward in 100-200m increments to as far as I'm going to shoot and get actual drops and then go back to my calculator and fiddle with BC in the appropriate velocity band until I get within .2mil. This kind of method has downsides since it is trying to model a smooth curved ramp using, essentially, stairs.

 

[yobuck]

Well whats wrong with confirming the data obtained from whatever source with actual shooting?
Or is that no longer allowed with modern day rules?

 

[Taylorbok]

Well whats wrong with confirming the data obtained from whatever source with actual shooting?
Or is that no longer allowed with modern day rules?

I thought that's what we were talking about.

 

[Eric Musgrove]

You can check scope tracking and level at the same time with a tall target test. I do it at 50 yards to check full scope travel. (110 moa is to tall to test at 100) 4x8 ply wood. Draw a plum line with an aim mark at the bottom. I draw a mark every full revolution of the turret, in my case 25 moa, (every 13" at 50 yards) shoot at the aim point and keep dialling up . If the bullet lands on the mark its all good, if it's low or high you must add a correction factor. If it starts to veer left or right scope isn't perfectly level

There is a great wright up on this posted here,

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