Ballistic Calculators and Subsonic Velocities?

mmattockx

mmattockx

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How accurate are the numerous calculators when working with bullets that are subsonic? If I have an accurate G7 BC for a bullet, does that accuracy follow down into the subsonic region? I'm not concerned with the transonic region, I am thinking more of subsonic muzzle velocities to start with.

Thanks,
Mark
 
Hi Mark,

The accuracy of the calculator is usually limited to the data being input... If you have accurate G7 coefficients then subsonic calculations will be mostly spot on.

Obviously the transonic period is somewhat unpredictable, but determining the path of a subsonic round will be just the same as a supersonic round.

iSnipe, in particular, has a limitation with regards to the muzzle velocity - in that it must be 500fps or higher to calculate... which is usually only relevant to airgun shooters.

Hope this helps clear a few things up.

Cheers,
bcode

[UPDATE: This post is not technically correct - see below for more info ]
 
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The usual G7 drag tables have several subsonic values. One bullet manufacturer program I'm looking at now has values for mach 0.0, mach 0.5 (this is why under 500fps is hokey), Mach 0.6 and so on. The one I use has 10 values less than 0.5 mach.
So you need to know what the velocity/drag data is for the program. G1 would be super-hokey if you just have one BC value, of course.
 
Juster said:
The usual G7 drag tables have several subsonic values. One bullet manufacturer program I'm looking at now has values for mach 0.0, mach 0.5 (this is why under 500fps is hokey), Mach 0.6 and so on. The one I use has 10 values less than 0.5 mach.
So you need to know what the velocity/drag data is for the program. G1 would be super-hokey if you just have one BC value, of course.
Click to expand...

I guess that is my question in a nutshell. Say I am using the G7 data from Bryan Litz's book, which was all developed at supersonic velocities. Is it still valid at subsonic speeds?

Mark
 
mmattockx said:
I guess that is my question in a nutshell. Say I am using the G7 data from Bryan Litz's book, which was all developed at supersonic velocities. Is it still valid at subsonic speeds?

Mark
Click to expand...
If you look at the "Drag and Ballistic Coefficient" graphs in the book, they are drawn from a certain number of points. You would have to ask Bryan how many points are subsonic. The more the better, clearly.
 
I started to second guess myself about this and so I did a little more digging.

Below is a graph from Bryan's book (p. 38) comparing his G7 coefficient with Doppler Radar data along with some actual points of impact. Notice the discrepancy between the G7 prediction and the Doppler data in the subsonic region... From the looks of this (and other data I just looked up), predictions will get less accurate as velocity diminishes (in the subsonic region, that is).

compared2.jpg
 
The other problem is specialty subsonic bullets have driving bands on them to reduce the friction and improve performance this is a benifit of true specialty projectiles .where as using standard over the counter bullets in a sub sonic application can cause some interesting issues as a bullet enters the transition stage. There is all so the issue of the bearing surface as the I stated the specialty projectiles have a smaller cross section/ bearing surface than standard projectiles.
 
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