Home Forums Hunting & Shooting Ask The Gunwriters Chronograph distance from muzzle?

Was wondering how much increasing the distance from the chrono effects velocity readings? In the morning my shooting building shades the chrono so I have it set up about 25 feet from the muzzle.

Lefty C

Most any ballistic software will correct it to the muzzle, the answer would vary depending on the initial velocity and BC of the bullet.

For me, I just use the number that comes out of Dr Oehler's box.

Ten feet from the rifle muzzle is pretty standard.

Steve

I started out putting it five big steps from the muzzle to avoid muzzle blast. It probably doesn't need to be that far away, but I continue to be consistant.

It depends on the powder 10 feet may not be enough.

I set mine up as far away from the muzzle as I can, about 15 feet away. Any closer and the muzzle blast removes the diffusers.

I normally go with 15 feet also. Any closer and the muzzel blast moves it around after a few shots.

lefty - I am NOT being smart. Follow the instructions of the chrono.

Steve IS correct. 10' is pretty standard.

Hopefully Dr. Ken O & Dr. Ken H will check in.

My Oehler 33 recommends 10' to the front screen. I'm sure others may have a diff. instruction.

Mine end up at ~15'. That equates to a 10 FPS difference from actual muzzle velocity according to JBM Ballistics calculations. 25' would be 17 FPS according to JBM. Anything inside ~12" and I start to see the screens getting moved around quite a bit from blast.

10' doesn't work with magnums most of the time. I end up with diffusers hanging off and have to straighten the box every 3rd or 4th shot. I usually put it closer to 15' but who is actually measuring??

If you set your screens too close to the muzzle, the rushing gas and shock wave from the muzzle blast can shake 'em enough to give you false readings.

A trick that I learned from Ken O is to use a baffle near the muzzle and shoot through a slot in it.

The baffle blocks the heavy rush of blast gas and shock wave that might otherwise shake the screens enough to affect the accuracy of the readings.

The baffle is simple � a piece of plywood two feet wide and four feet high, set and propped more or less vertical on the ground, with a two-inch-wide vertical slot in the middle to aim and shoot through.

The difference between actual muzzle velocity and chronographed velocity, IMHO, isn't great enough to worry about � as long as you always set your screens the same measured distance from the muzzle every time you set 'em up.

As close as ten feet with a baffle or as far as twenty-five feet without the baffle wouldn't give me heartburn, but I'd start to worry about any greater distance.

Don't get hung-up on too-close attention to the numbers themselves. They vary anyway. Don't sort barn rafters by widths measured with a vernier caliper.

Consistency is key. I put a piece of tape around the cord that goes to the readout at 12' and use it to measure the distance between the muzzle and chrony...

As a general rule we suggest the first screen 10 feet from the muzzle for our systems. We consider that the velocity lost in the 12 feet or so to be insignificant in comparison the the muzzle velocity or the actual measured "instrumental velocity". You may get pleasure by adding 8 or 12 fps to all velocities before you compare them, but I'm lazy.

You should be more worried about the effects of flash and blast. Nasty things happen near the muzzle and as friend Ken Howell mentioned, a blast shield hides many sins. Remember that the really nasty effects of blast are when you have a subsonic bullet preceded by a blast wave traveling at the speed of sound. That blast wave often fools the skyscreens before the bullet gets there.

Thank You Dr. O !

I know all chronos are not created equal, but as in my M 33, the bullet passes OVER the skyscreen, not through any aparatus. I spaced my screens 5' apart AFTER the front screen space from muzzle of 10'. Therefore my velocity is read at 12 1/2' from the muzzle.

I'm not familiar with the M 35 as to what is looks like but I've had my 33 @ 30 yrs. I hope it lasts as long as I need one. It has been totally reliable.

Ken O,

Is there a trick or tip to chronying shotshells?
I've tried everything I know of to get consistant readings, but I've all but given up.

What do the factories use to get their velocities?

Thanks for the help,
Elk Country

Not being an expert at all, but muzzle is about 4 - 4,5 yards from the cronograph. Closer than that gives many "error".

Measuring shotshell velocities is much more difficult that rifle or pistol. The shot string is continually stretching out and the individual pellets are small. Our suggestions are to use the most open choke available, put our start Skyscreen III three or four feet from the muzzle and use a spacing between screens of four feet. You can protect the guts of the first screen with a block of soft wood to pick up stray pellets or wads. Anything that would stop a direct hit can also splash lead back into your face.

We believe that using photoelectric screens to measure shotshell velocities can give you good comparative data, say between you load and similar Winchester AA, but you may not get the same average velocity as is measured by the factory procedure.

The factories use a coil system designed and built by Oehler almost forty years ago. This system tends to ignore the stringing pellets in front of the main charge.