Chrono Question

[spdrman]

I've been reloading for a little over 4 years now and have never shot thru a chrono until this week, in the past id just work up different loads until id find the most accurate load and never worried about how fast they shot, I purchased a Savage LRH 7mm a couple months ago and have found a couple loads that shoot great so I borrowed a friends chrono and was just going to use which ever load shot the fastest since they all group the same on paper, when I shot thru the chrono I had a variance of 300 fps between 9 shots of the same load, I measure out each load individually and know they are all the same, so I grabbed my AR15 and my bolt action 223 and shot them thru the chrono and only had a variance of 12fps between the 2 guns and I use the same process to load for all my guns, any ideas why my 7mm is so inconsistent

 

[bman73]

how close to the chronograph were you shooting? A 300 fps variance seems far too extreme to me to be consistant POI. If I had to guess I would say that the chrony was being affected by muzzle blast from the bigger gun to get that big of a difference. When shooting my bigger guns ( well big to me anyways) I have had the chrony out as far as 30 ft to get a proper reading on it. Hope this helps.

 

[spdrman]

I had it about 2 feet in front of the barrel guess I had it to close ill have to try it out farther thanks

 

[kcebcj]

I had it about 2 feet in front of the barrel guess I had it to close ill have to try it out farther thanks

At least 10 feet with the 7mm and that's close. I have a permanent shooting bench and the mount for the chronograph is 16 feet from the bench.

There is a formula to compute the velocity loss from muzzle to chrono. At 16 feet with my velocitys I needed to add 9.6 fps

 

[Dean2]

As said, definitely too close, that is even to close for the 223. I use 15 feet as my all around distance. Works for magnums on down and the add is consistent that way.

 

[Gene]

If you are using a Chrony that folds in half, make sure both halves are level. If not, you will get erratic readings.

 

[Eaglet]

Yup, you're problem is not a problem!

These are my personal distances that work for me:

None Magnums ------- 10 ft
Magnums without Muzzle brake--------12 ft
Magnums with Muzzle brake----------15 ft

When I set up my chronograph, before I shoot the big boys I run some
0.22 LR bullets through to make sure all is fine. My chronograph will read my 0.22's and the pellets from an air gun at any of those distances.

Good luck!

 

[boomtube]

"..definitely too close, that is even to close for the 223. I use 15 feet as my all around distance."

Definitely so. I carry a 15" length of (bright colored ribbon) in my range box to space my first screen from the bench for consistancy's sake. The further out we place the screens the more likely it is we will shoot them so consider that in your tripod placement.

Correcting for muzzle velocity is pointless, the difference is too low to bother with.

 

[spdrman]

Thanks for all the replies, wish I had time to get out an shoot soon but I wont be able to make it out again until the 12, I was going to have a custom gun built thus spring and the gun smith was trying to talk me into a 7wsm because it has more consistent muzzle velocities than a 7mm but shortly after talking to him I find out im having my first kid and my wife said id have to wait on building a custom gun, is there any truth to the muzzle velocities being more consistent on the short mag, what would you guys consider for the bare minimum in variance for long range shooting, I think with my current load it will do just fine, I can shoot under 3/4 inch groups at 200 yards and last week I shot 6 rounds at 1042 yards and my group was about 9 inches vertical but about 2 feet wide due to the various wind speeds im still learning how to shoot in. Thus far im more than happy with my stock factory rifle

 

[Eaglet]

"
Correcting for muzzle velocity is pointless, the difference is too low to bother with.

You're right, but since I still have to punch it in the ballistic program I might
as well just add 9 ft/sec to the chronographed velocity for my 338 Edge. Now
those 3" difference at 1200 yards, though undetectable, I know they're not
putting me farther away from my objective.

 

[joe0121]

Does using a muzzle break allow you to get the chrono closer?

 

[MNbogboy]

Everyone here seems to agree on the distance (my Chrony stand is set up at 12 feet)....Another thing to consider is bright sun and sunshine angles....I have had readings 150 to 200 fps apart on sunny days in the past with the only explanation being chrono error.....

The chincy sunscreens that came with my first chrony and now the second one don't always work when the sun shines wrong...For years I quit using it on real sunny days until I fashioned a much larger shade with a piece of 1-1/2" x 16" x 2 long styrofoam...It is translucent enough to let light in but you can actually "shade" the chrony at almost any light angle by lowering it close to the bullet path (it slides on 4 uprights that are fastened to the stand).....Cloudy days you don't need it at all....Readings have been very consistent with both cloudy & sunny day readings since i started using the "big shade"...

I don't have a picture of the stand with the uprights installed, but the picture enclosed will give you an idea that the 4 uprights fasten to the two horizontal frame members and are about 16" tall....The styro sheet slides up and down on them.....