Questions about Ballistic FTE

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by Skinny Shooter, Mar 18, 2011.

  1. Skinny Shooter

    Skinny Shooter Well-Known Member

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    Just started running it on an iPhone 3G and its pretty slick.
    Especially the ability to take atmospheric conditions at the time of your "Zero" and then reset the calculations to match the current atmospheric conditions.

    Am trying to figure out how to use the drag model functions.
    If I have a manufacturer published BC do I set the Native Drag Model to G1?

    Which Drag Model (not the Native Drag Model) should be used for the following bullets:
    Nosler .224 55gr BT (published BC of .267)
    Hornady 6mm 87gr Vmax (published BC of .400)
    Hornady 6mm 105gr Amax (published BC of .500)

    Do I use the G5 Drag for the Nosler which is the drag model for a low based boattail.

    G7 is for a VLD and don't think the Ballistic Tip fits that profile.
    Would the G7 Drag fit the other two?

    On a side note I've noticed that while using drop charts created with the G1 model in years past, the bullet strike was always a bit low.
    Running numbers with G7 seems to take that extra drop into account/
    thanks,
    Allen
     
  2. James H

    James H Well-Known Member

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    Unless the source for the BC number tells you differently it will be a G1 BC. I think Berger is the only one that publishes BCs other than G1 and they are G7 but are stated as such.
    If you purchase Bryan Litz Applied Ballistics for Long Range Shooting book G7 drag coefficients are published in it for many bullets and are better suited for VLD bullets.
    book

    James
     

  3. groper

    groper Well-Known Member

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    All manufacturers quote G1 BCs for their bullets, only some quote G7 in addition like berger for example.

    So, G1 is almost always the "native" BC in ballistic FTE... that said, the reason you never get a trajectory to work with published G1 BC`s, is because they always invariably publish the G1 BC @ or near a muzzle velocity typically somewhere around 2800-3200fps. The problem is that G1 BC`s are heavily dependent on velocity.

    So when you shoot at something and notice your POI is low when using a manufacturers G1 BC, is because the bullet is continuously slowing down after it leaves the muzzle, and the manufacturer G1 BC no longer is relevant.

    It is inappropriate to use muzzle velocity G1 BC`s (for any bullet regardless of its shape) in a trajectory calculation because the appropriate BC to use is one that "cumulatively averages" the BC over the course of its trajectory and the entire velocity regime which this encompasses. Unless you have a cumulatively averaged BC, it wont matter which BC model you use in ballistic FTE, they will all be inaccurate...

    Id recommend this site to learn more about it Home and Bryans book which covers many subjects related to this matter.
     
  4. Skinny Shooter

    Skinny Shooter Well-Known Member

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    Thanks for the replies.
    Is FTE useless then?
    Am I understanding correctly that FTE does not average the BC of a bullet as it loses velocity?
    That book looks like a great resource and will purchase one sometime down the road.
    Unfortunately, around here I don't have a place to shoot and get my drops that way.
     
  5. James H

    James H Well-Known Member

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    Bryan's book would be very helpful to you, after reading it you will better understand how to use Ballistic FTE. I say this because based on your last comment I think you're frustrated a little with it. The book is worth every penny. I purchased 3 copies form Bryan, gave one to a friend and keep one at the office and one at home because I use it as a reference often, it's that good.

    James
     
  6. groper

    groper Well-Known Member

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    No, FTE is brilliant - i use it myself...

    FTE doesnt know how to average the BC, you have to give it the averaged BC in the input. No ballistic calculator can average the BC automatically as it does not have all the information it needs in order to be able to do this.

    Different shape bullets and their relationship with BC velocity dependence, vary at different rates due to their aerodynamic shape and mass.
    So if we have bullet A - with a BC of say 0.5 @ 3000fps and BC of 0.4 @ 2000fps...
    this doesnt mean that bullet B, still with a BC of 0.5 @ 3000fps, will have the same BC as bullet A @ 2000fps... bullet B might be 0.3 @ 2000fps.... so the decay rate is variable from bullet to bullet.

    in other words, the BC velocity dependence is unique to every single different bullet. So unless the ballistic computer can recognize a multiple velocity bracketed G1 BC system - such as sierra quotes for their bullets as a means of helping this very dilemma- then you have to derive an averaged number yourself or obtain the averaged number from someone else who has hopefully done it accurately for that bullet, in order to have the best fitting trajectory prediction curve possible.

    This is the service Bryan Litz provides, and he provides it in an averaged G7 BC format for long range bullets (which we are most interested in) as G7 is much less velocity dependent than G1, the drag curve fits better to long range bullets, and provides a much more accurate trajectory prediction when used in a ballistic calculator. This is a useful service because manufuacturer muzzle velocity BC`s are useless for anything but a rough comparison of bullets -as in which bullet might have lower drag than another, and of course a selling point for marketing purposes.

    When long range shooters talk about BC and its use in trajectory prediction, what we are really talking about is "cumulatively averaged BC" sometimes referred to as "real world BC", not whats written on the box of bullets...
     
    Last edited: Mar 18, 2011
  7. ctressler

    ctressler Member

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    I second that, great book, great tables. Sometimes a G1 is better than a G7 and Brian's book indentifies these.
     
  8. ctressler

    ctressler Member

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    FTE can accept multiple velocity bracketed BC's. It has several stored in its memory and the software author just updated it to accept user define-multiple BC values.
     
  9. orangeride

    orangeride Well-Known Member

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    I thought fte could convert g1's to g7.
     
  10. ctressler

    ctressler Member

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    It does convert to G7 if you ask it to. However, I noticed the other day when I did this it took a multiple velocity bracketed G1 BC and converted it to a single G7 BC. This could have been an error on my part. The single G7 BC did put me right on though.
     
  11. orangeride

    orangeride Well-Known Member

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    Sorry if it seems like I'm changing the subject, but I've got another question with fte. When I zero the bullet drift from spin, fte also zero's the wind drift. When I physically sight in the rifle at 300yrds the spin drift is zerod. When I run fte and i throw in a 10mph 90deg wind it still shows no drift at my 300yrd zero. If I turn off the drift zero setting everything looks ok, but I know it's not taking into account the spin. Does this make sense to Any one, or do I just sound nuts. Maybe I'm just missing something . Thanks
     
  12. ctressler

    ctressler Member

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    I realized the very same thing last night. I think my work around will be to dial the spin drift in my scope as part of my 300 yard zero and set the drift zero setting to off.
     
  13. orangeride

    orangeride Well-Known Member

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    That works for your 300yrd zero, but if you've got a 750yrd shot and a left to right 5 mph wind and you've got a right hand twist you have a pretty
    Big variable on your hands. I think the best thing is to run the fte to find the spin drift with it being zerod, then run fte to find the wind drift not zerod. I wish they'd come out with a up date. What do you guys think?
     
  14. ctressler

    ctressler Member

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    orangeride your right, what I mean is I will leave the zero setting "off" and just zero my rifle with the spin drift included in the zero for 300 yds.

    I don't think you need to run your shot twice through FTE for wind and spin drift, unless you want to know what each is separately.

    I've been thinking a lot about the wind direction and I think FTE makes since if one punches in the direction a flag is blowing. So in a headwind the flag is actually flying toward 6:00 and a left to right wind the flag is flying toward 3:00. This convention seems to make the most since and everything comes out right with the FTE.