Adding a skootch to the chrono readout?

[Alan Griffith]

When pulling an average velocity reading from your chronograph and taking that velocity to input into what ever ballistic app you're using, do you add a skootch more to make up the loss of speed from the muzzle to the chronograph? Someone once told me to add about 30 fps so that if my chronograph reads 2750 fps I should input 2780 fps in my ballistic program. This when the front screen of my 35P is 10' from the muzzle. Just this morning, shooting at 950 yds with a 140 Hybrid from my 6.5x47 I was impacting several inches low. This rifle/load is currently placing 5 into less than 1/4 to 1/2 moa at that range so I'm pretty sure of the drop/impact distances.

Alan

 

[royinidaho]

Shooter and JBM take chrono distance and velocity and show the "zero" distance (muzzle) velocity.

I tend to use whatever velocity that makes the drop chart work up to a point at which I fudge a bit on bc also.

Takes a bit of shooting but is worth it.

You shouldn't have much problem with the accuracy of your rifle.

 

[Alan Griffith]

Shooter and JBM take chrono distance and velocity and show the "zero" distance (muzzle) velocity.

I tend to use whatever velocity that makes the drop chart work up to a point at which I fudge a bit on bc also.

Takes a bit of shooting but is worth it.

You shouldn't have much problem with the accuracy of your rifle.

Roy,

I thought I was pretty familiar with Shooter but I'm not sure what you mean by "take chrono distance and velocity and show the "zero" distance (muzzle) velocity". Will please expound on this issue?

Alan

 

[royinidaho]

Allen,

Good catch!

This is a case where "assuming" made an *** out of me.

I simply haven't thought about how Shooter handles distance to chrono.

My process is to place the chono @ 15 feet and run the data on JBM Ballistics, which gives the velocity @ the muzzle based on distance to the chrono.

I then use that corrected velocity in Shooter. Then fudge it as needed, along with bc, to Shooter to match actual drops PDC (pretty darn close).

I think this is another point in favor of the Magneto chrono thing or the acoustic gizmo down range.

Thanks

Roy

 

[stomp442]

If you use the G7 b.c. you shouldn't have to fudge it if any. The G1s are very velocity dependent and can cause inconsistencies in your drop chart at extended ranges. Another important aspect of using ballistic tables is making sure that your using your actual conditions like altitude, temp and humidity. When the data is properly inserted the output data should be within a click or two. Some programs only give you the standard sea level solution which can be way off if shooting at higher altitudes. Also day to day changes in temp can affect trajectory. Say for example you shot yesterday at 950 yards in 90 degrees and needed 14 moa and today at 950 yards in 70 degrees you might need 14.5 moa to achieve the same poi. Small differences like air density and humidity make considerable difference at longer ranges. Also an accurate scope height measurement is essential in accurate drop charts. Just some ideas as to why things may not be working out right.

 

[Alan Griffith]

Roy,

Now why didn't I think of that. I usually set my first screen @ 10' which means my middle screen is about 12-13'. I simply looked at the 10' and 15' loss in velocity and extrapolated a 12-13 fps loss. Add that to the 2750 my 35P is telling me and BAM. I'll likely get back out to my 950 yd spot either Friday or Saturday am.

Stomp,

Using G7. Getting all my "on the spot" weather data from my Gunwerks G7 and when I pull the data off of it, I then input that data into my Shooter program. The G7 and Shooter are always within .1 moa. Thanks for the tip anyway but way ahead of you. I'm no bouncing turnip.

Alan