#1-Why are you supposed to set the chrono up 10 feet from the end of the barrel ?
#2- How do you set it up to be sure you will not hit it with the first shot from your gun ?
Any and all ideas for the set up will be appreciated !
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#1-Why are you supposed to set the chrono up 10 feet from the end of the barrel ?
#2- How do you set it up to be sure you will not hit it with the first shot from your gun ?
Any and all ideas for the set up will be appreciated !
10 feet from the muzzle is so that the expanding gasses from the gunpowder dont give you false readings. The "shockwave" from the gasses is moving a fair bit faster than the projectile for the first few feet.
When I set up to shoot over a chronograph I will set a target up downrange, then set a bolt action rifle on bags on the bench. Remove the bolt and sight in on the target, and without moving the rifle then set up the chronograph. Now get behind the rifle and look down the bore. You should see the target centered in the barrel and hopefully not see any part of the chronograph.
Sorta like that...Attachment 194452
Pipefitter and OS OK have pretty well covered it. Ten feet is a number that a lot of people use because it works. By setting the chronograph screens the same distance each time means that your results from each trip to the range will match. If you set up 7 feet one time and 15 ft the next the results will be slightly different. Take a look at TM 43-0001-27 (https://www.ar15.com/content/webPDF/TM43-0001-27.pdf) and you will see the Army has used many different chronograph distances for various rounds.
As your testing keep an eye on your chronograph screens to make sure they aren't moving. For most people the setup up is mobile using a tripod or something. This is not "rock solid" and wind or "muzzle blast" can sometimes cause the screens to creep out of alignment. Along with not properly tightening a screw or lever :oops:
A great tool and a lot of fun.
There is the LabRadar setup, pricey but just sits on the bench, use radar to measure bullet speed $500 plus. I have a MagnetoSpeed, hangs off the rifle barrel, but limited to longer single barrels. I would choose the LabRadar over the MagnetoSpeed in todays world. Technology just keeps moving, I do have an old Chrony for handgun and tube-fed leverguns.
You might still get "false" readings at 10' - I set mine at 15'.