Velocity question.

[MT257]

I have been working on a load for my 243 and 70 grain Nosler Ballistic tips. I chose IMR 4064 for a powder. I started at 38 grains and increased to Nosler Manual max load of 41.5grains. My question is that after testing the best load was the max load of 41.5 grains. I chronographed these loads and averaged 3600 fps. But the manual shows they averaged at 3478. I set the COAL at the length in the manual. I have attached the group of this load as well. What causes this spread in velocity and should I be worried. No signs of pressure is noticeable.

 

[FearNoWind]

Not sure what you means by "what causes spread in velocity ..." Muzzle velocity is unique to every rifle and every load. Velocity references in manuals are only a guideline.
Also not sure what you mean by "No signs of pressure ..." It's possible to exceed safe pressure limits in a rifle without showing "signs". Yeah, I know, I've read all that sfuff too. But by the time your rifle shows "pressure signs" you're most probably waaaay over safe pressure limits. Will the gun blow up? Sometimes ....

http://www.fieldandstream.com/blogs/gun-nuts/2013/05/big-bang-theories

https://www.youtube.com/watch?v=IkXfYMkTNNs

https://www.youtube.com/watch?v=ZURyxaSdL3g

 

[tbrice23]

About 20 different things could cause velocity differences.

Actual bore diameter difference,
Powder lot variations,
Primer lot variations,
Altitude,
Temperature, .....

 

[rcairflr]

Don't forget inaccuracy of the Chrono

 

[red dawg]

Also test barrels not test rifles

 

[MT257]

Spread meaning why my velocity is higher than the manual. And pressure sign goes as the primer is a little flattened. And I guess I have a shooting chrony in the gamma model I'm not sure how consistent this is. But directly following the 243 I chronographed my 300 and in three shots my difference was two fps total.

 

[rcairflr]

Spread meaning why my velocity is higher than the manual. And pressure sign goes as the primer is a little flattened. And I guess I have a shooting chrony in the gamma model I'm not sure how consistent this is. But directly following the 243 I chronographed my 300 and in three shots my difference was two fps total.

That is what I was referring to with my comment on the accuracy of your chrono. There is no way to check the accuracy short of buying a very good quality chrono or LabRadar. It could easily read 150 fps slow or fast, maybe even more. To be absolutely accurate, it must be build to very precise measurements and components. As built, it should be repeatable, but not necessarily accurate for velocity.
So my point is, even though you read 3600 fps, doesn't mean the bullet was actually going 3600 fps.

 

[Rich Coyle]

I loaded a .22-250 for a friend in Arizona. The accuracy load was one grain over max in the Nosler manual. To make sure the load was okay for Arizona I put a box of twenty on the dash of the car with the windows up on a sunny day here in Oregon. They worked the same as a factory load at normal hunting temperatures.

I don't see a problem with your loads.

 

[MagnumManiac]

I have seen chrony readings that were 400fps over published velocities. When the same loads were checked on a different DAY with the SAME chrony, they were back to where they should be.
Some chrony's are suseptible to bright sunlight, even with the diffusers attached.
No chronograph is infallable, I have had erroneous readings from my Oehler 35P on occasion in bright sunshine.
It appears the Chrony brand prefer a bit of cloud cover to function correctly, also, make sure it is fully opened, even a small difference in opening length will result in high readings, as the screens are cloer together.

Cheers.
gun)