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Old 16th April 2015, 08:53 PM
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Brian.Samson Brian.Samson is offline
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Join Date: Jun 2009
Member of: Pontefract, Doncaster Airgun Range
Location: Doncaster
Posts: 2,324

Originally Posted by Tench View Post
Brian I have measured the velocity of 8.44 Exacts at the muzzle and down range achieving a bc of .0267 over 27y in doors using my Hornet. I have also plotted the trajectory of my Gold Star by measuring from the cross hair. I drew crosses on a box and labelled then 8, 10, 12 etc to 45 yards. I put the box at the measured distance and fired 3 shots with the crosshair placed on the cross, I did this at each distance (measured) and arrived at the following measurements for the poi from the cross hair. The scope height is 1.6" JSB exact 8.44gn @ 784 fps.
8y -11mm
10y -5mm
12y 0
13y +2mm
15y +7mm
20y +15mm
25y +21mm
30y +23mm
35y +21mm
40y +7mm
45y -5mm
Now plotting this in CG and juggling settings to make it fit requires a bc in the region of that measured with my Hornet and it still doesn't quite line up with the actual achieved poi's but this is the trajectory the rifle is shooting. It would also seem the actual optical line of sight is slightly above the measured scope centre as using 1.7" aligns the poi's better.
Doing it this way eliminated scope magnification and reticle spacing errors.
All useful information, but as Conor says, ideally you want to be downrange testing at 50+ yards.. 50m is even better.

The reason I say that is because when I test pellets even a bad batch of 8.4's will group nicely at 30 yards, when you get out to 50 yards that's when it's easier to spot a bad grouping batch.

My theory on this is that as the pellet slows any instability is more pronounced.

Also be careful with measuring your scope height - it's not the scope height that ballistics software is asking for, it's line of sight height at the point the pellet leaves the barrel when the rifle is shot. So you can't ever measure it accurately when the gun isn't being shot.

It's easy to look at a barrel and assume that a steel tube remains perfectly still, when in fact there are often slight oscillations when the pellet travels down the barrel.

The closest you'll ever get practically to an accurate measurement is by using the tin foil method - I've described it on here a couple of times, but I don't have the time to do a search for it at the moment.

The tin foil method eliminates things like barrel droop, mount packing, adjustable mounts, barrel harmonics and muzzle flip, although it's not perfect, it's still the most accurate method.
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