Got a ballistic question

[potatoe]

I've got a good shooting .284 win in a tube gun. My issue is that any of my ballistic charts are making me hit high once I get past 350 yards sometimes being up to a full MOA off. On paper it does great out to 300, my clicks are correct for going from 200 to 300 yards. The tube gun does have a high scope to bore ratio but I have that put in the ballistic calculator. The velocity is around 2925fps according to my beta master chrony.

Any suggestions on what could be going on? I think I might just have to shoot out to 500 on paper (hopefully find somewhere I can do that) but cant figure out why the ballistic calculators are this far off.

 

[chkunz]

I've got a good shooting .284 win in a tube gun. My issue is that any of my ballistic charts are making me hit high once I get past 350 yards sometimes being up to a full MOA off. On paper it does great out to 300, my clicks are correct for going from 200 to 300 yards. The tube gun does have a high scope to bore ratio but I have that put in the ballistic calculator. The velocity is around 2925fps according to my beta master chrony.

Any suggestions on what could be going on? I think I might just have to shoot out to 500 on paper (hopefully find somewhere I can do that) but cant figure out why the ballistic calculators are this far off.

Have you read "Applied Ballistics for Long Range Shooting" by Bryan Litz? You did not give a lot of detail with your question, what bullet, what BC are you using, etc. but the book will help you answer a lot of questions. Also one basic input is muzzle velocity and there is a lot of uncertainty with most folks on making that measurement. But after all is said, if you are within one MOA that sounds like reasonable results.

 

[potatoe]

No i haven't read that (I really should) its a 162 A-max bc .625. I have a decent amount of info put in the one chart. Baro. pressure, alt. temp etc. its just kinda p!$$ me off missing a ground hog once I get past 400 because of the high shots. I know the obvious answer is to take a few off but that starts a bunch of uncertainty for the longer shots and just want to make sure I'm not missing an answer starring me in the face plus these calculators have been fairly accurate before so I just want to cover my bases because you guys have some great input

 

[chkunz]

The BC you quote is probably a G1 and the bullet you are using probably better matches the G7. This is explained in the book. I have read the book and I stayed at a Holiday Inn once.

 

[mikecr]

Well when you mention GH shooting, I wonder about your accounting for slope and level.

 

[potatoe]

The scope is level and I measure the slope with an inclinometer (?is that spelled right lol) from my phone that I have compared to an inclinometer from my work and it was with in .5* so I think is fairly close. That being said I have change the angle in my Bal. Calculator 30* to get a difference of 1 moa at 400 yards.

T-rex: I just going off the manf. published BC. But I don't think that your right (if I'm understanding what you meant), if my BC is lower than what is being calculated then the bullet would have more drop not hitting higher.

 

[chkunz]

The scope is level and I measure the slope with an inclinometer (?is that spelled right lol) from my phone that I have compared to an inclinometer from my work and it was with in .5* so I think is fairly close. That being said I have change the angle in my Bal. Calculator 30* to get a difference of 1 moa at 400 yards.

T-rex: I just going off the manf. published BC. But I don't think that your right (if I'm understanding what you meant), if my BC is lower than what is being calculated then the bullet would have more drop not hitting higher.

Your logic is sound but I did not say that the BC you are using is too high or too low just that you appear to be using G1 and you are using a bullet that is closer to the G7. That is the main point of Brian's book, the manufacturers and many of the ballistic programs use the G1 but our long range bullets that we are using more closely match the G7. Bryan goes into great detail on the problems that this causes for folks like you that are trying to get much closer with their calculations so that the first shot is closer to the aim point. This may not be your problem at all but it is worth a look. Any way you can learn a lot from the book. I still think that our ability to measure muzzle velocity is a significant source of error but I am in the minority on this topic on this forum and have less experience with the chronograph that the other folks who have faith in their measurements.

 

[potatoe]

Thanks T. I have heard great things about the book (I do really want to read it) and think I need to get some more knowledge on the long range effects. I was also leaning on the chrono as the weak link but once again (like the slope angle) my margin of error has to be great to make such a change in the POI according to the ballistic calculators. But this has only been the case on a few targets so maybe I'm overlooking something and the gun shoots great on paper out to 300 so I'm guessing its the driver.

 

[gme]

http://www.accuracy-tech.com/truing-ballistics-data/

 

[potatoe]

Thanks Gary! I believe I might have to put it on paper out to 400+ and tweak my info till I get a chart that fits the rifle. Its just tough to get a place to shoot that far.

 

[Bryan Litz Ballistics]

Good link and info Gary. I'll add one thing to the linked article which is often overlooked, and often the true culprit for predictions not matching reality: actual scope adjustment.

Sometimes when you dial 30 MOA (for example) you only get 28, or 31.5 and you don't know it. When you hit high or low, you automatically assume the ballistic prediction was in error when in fact the prediction may be spot on and the scope didn't apply it as intended.

Just something to add to the list of things to check for those who are trying to resolve errors in predicted vs. actual trajectory.

-Bryan

 

[FatBoy]

I used to get wrapped up in this when UKD shooting appealed more to me. I'd use jbm to get close, but I take my rifles to 300, 600 and 1000 to verify dope and record temp, light, wind etc. I have found it to accurate to within 1MOA to 1k even with G1 numbers, but that kind of accuracy won't get a whistle pig at say, 550 yards.

Only way to know is to go and verify at distance. If you use the same stands, make a range card with target reference points and come ups recorded so you can quickly enter the adjustments or hold offs.

Brian brought up an excellent point. Have you box tested your scope? I have had to send an NXS back twice (they replaced it free after the 2nd return) as it hit one section of the elevation range where 3moa moved the POI 4.5" up and down, repeatable. This same scope drifted windage on the way down from 35moa BUT it was totally repeatable. As long as you know, not a real issue but I couldn't stand it.