Calculating P.O.I.

[BCB]

Is there some program on the Net that will allow me to calculate points of impact if I don’t know the zero…

That is, my bullets are hitting ~-.75” at 100 yards. I have tried to work the numbers backwards but I never get a zero calculated with the -.75” hit at 100 yards…

I could shoot it at various locations, but I know where my bullet is impacting at 100 yards and a calculator would make it easier...

Any help?...

Thanks…BCB

[petroid]

there are many ballistic calculators on the web. you need to know velocity, ballistic coefficient, and sight height. I have an app on my phone for it.

[John Boy]

BCB - if you know the velocity of the round (or a get close), here's a free trajectory program ...
http://bulletin.accurateshooter.com/2008/02/free-point-blank-ballistics-software/

[BCB]

BCB - if you know the velocity of the round (or a get close), here's a free trajectory program ...
http://bulletin.accurateshooter.com/2008/02/free-point-blank-ballistics-software/

I'll check it out. Do you have it installed on your computer?...

My impact at 100 yards is -.75". My 'scope height is 1.6". So, it is possible the bullet's flight never crosses the line of sight. I think this is possible, but I am not sure. Therefore, I would not necessarily have a zero according to the placement of the crosshairs. The bullet's path might always be below the line of sight...

So, that being said, my bullet is .277" (6.8mm), a BC of .400, and a muzzle velocity of 2450 @ 10 feet...

A dang good shooter this little 6.8mm SPC is out to 350 yards...

But, not a cast shooter...

Thanks...BCB

[fouronesix]

At -.75" @ 100 it doesn't appear that your bullet will cross the line of sight (all numbers in accompanying table are negative). The only data I didn't have was the bullet weight- so I used 130 gr. as a default. Using the numbers you posted, here is the closest I could come to your current ballistic profile with the sight setting you have, namely -75" @ 100. You can go to this site and play with the numbers. And no this calculator is web based so you don't have to load anything into your computer.
http://www.jbmballistics.com/cgi-bin/jbmtraj_simp-5.1.cgi

[MtGun44]

Never had a really good correlation between what the ballistics calculations said
and reality. OK, just not dead on. Like 1 or 2" off at 200 or 300 yds from predicted.

Useful, but really, you need to shoot it to see.

Bill

[petroid]

Never had a really good correlation between what the ballistics calculations said
and reality. OK, just not dead on. Like 1 or 2" off at 200 or 300 yds from predicted.

Useful, but really, you need to shoot it to see.

Bill

Youre absolutely right. You do have to verify zeroes and poi at different ranges. The calculators are a good estimator that can give you a rough idea of what to expect though

[44man]

Never had a really good correlation between what the ballistics calculations said
and reality. OK, just not dead on. Like 1 or 2" off at 200 or 300 yds from predicted.

Useful, but really, you need to shoot it to see.

Bill
Correct, I would set up targets every 50 yards to 600 and shoot each target once zeroed at the range I wanted, in my case it was 350 yards with the Swift. Then I wrote drop on paper and what scope adjustment I needed, taped it to the scope.

[BCB]

I reckon I might have to shoot it at various ranges…

The reason I wanted to use a calculator is the fact that the ‘scope is set for 90 and 110 grain bullets. These two bullets have amazingly close points of impact out to 250—that’s about as far as I will ever shoot this 6.8mm SPC…

I really don’t want to move the vertical on the ‘scope up and down for the 90/110 when I am shooting the 120’s…

I know ‘scopes are to return to where they were after moving the crosshairs, but I simply have never found this to be exactly the case…

So, I guess I might load a few 120’s and head to the range…

I wish there would be some type of program that would allow me to enter the -.75” impact and 100 yards and it would do the rest. I really only need “close” P.O.I.

I have messed with the online calculators and I cannot get a -.75” point of impact at 100 yards. I still think the bullet never crosses the line of site?...

Oh well…

Thanks…BCB

[Larry Gibson]

BCB

Understand that even if the calculator is 100% correct and you shoot very well the POI is only for the center of the cone the group at the range shot at. It does not tell where the bullet will hit within that cone of fire. The cone of fire gets larger as the range increases and is generally linear in dispersion out to roughly 600 yards. Throw in humidity, elevation change, uphill/downhill angle, actual velocity of the shot, actual BC of the bullet, actual height of sight above line of departure, wind velocity, wind angle, variation of wind across the distance and several other variables and you quickly find most any program only gets you close. Unless you have a portable computer that you can input all the correct info into how close is "close" is also variable. Even if you go out and test the actual drop the conditions where you'll be shooting will be different.

The skill and experience of the shooter in judging the variable conditions when making a long range shot can not be compensated for by any equipment or computer program. However 250 yards, even with the 6.8 SPC isn't really a long range shot with 120 gr jacketed bullets at speed unless you are shooting very small targets. You might find a 175 or 200 yard zero more practical and just use a little hold under and hold over for the other ranges if need be.

Larry Gibson

[C. Latch]

I reckon I might have to shoot it at various ranges…


You have just answered your own question. There's no substitute for shooting.

[Love Life]

Are you saying that your bullets are impacting .75 low from your POA at 100 yds?

[44man]

Larry says it good. Shooting IHMSA for years and reading wind had me aim off to hit but I never changed windage, not ever! times I had to aim at the next ram to hit mine. Hold over is the same if you know your gun. You just must shoot to see. I never found any paper work to match.

[fouronesix]

BCB,
Here's another calculator that provides a real time visual trajectory you can play with- if that is more to your liking.
Be sure to click on "Imperial" and to enter data to define your own bullet.
http://www.norma.cc/us/Ammunition-Academy/Ballistics/iframe/

No matter- calculator or not, I still can't figure why a sight setting of -.75" @ 100 yds is set in stone and is somehow magic for setting up for some kind of rattle battle tactical match shooting out to 250 yards with that cartridge. The scope is adjustable for vertical isn't it? And can be returned to -.75" @ 100 yards if you wish, right? Seems like a sight setting somewhere a little high @ 100 yards (maybe 2"-2.3" high @ 100)) would provide the least amount of "above" or "below" POI out to 250 yards. Trajectories are parabolic curves. With the ballistics of that cartridge and the type of targets shot, seems that type of sight setting would provide almost a "point blank" capability out to 250 yds.

[BCB]

Are you saying that your bullets are impacting .75 low from your POA at 100 yds?

Yep, as I have not readjusted the vertical on the 'scope. It is set for the 90/110's and I really don't want to keep changing that setting every time I shoot the lighter bullets and then go to the 120's...

Thanks...BCB

[Love Life]

So it is hitting where desired with the 90/110 gr bullets, but .75 low with the 120 gr bullets at 100 yds?

[C. Latch]

What brand of scope are you using? Most better scopes can EASILY handle going back and forth an inch of elevation as you switch between loads.

[BCB]

What brand of scope are you using? Most better scopes can EASILY handle going back and forth an inch of elevation as you switch between loads.

I have a $500.00 Burris Variable...

I think it's a pretty good 'scope...

BCB

[C. Latch]

I have a $500.00 Burris Variable...

I think it's a pretty good 'scope...

BCB


Then your fears are unfounded.

Check zero with your 110 loads, then shoot the 120s, note the difference in POI, and keep a notebook of when you adjust from load to load. Or if the difference in POI is only vertical and only minor, just make up a chart for each load and tape them to your stock.

But....why the back-and-forth? You could just standardize on one load and save this headache.