confusing velocity change

[chet]

okay guys and gals.... explain this one to me....

one week ago, I tested a load in my 308
44.5g Varget 150g ballistic tip
sprayed a 4" group
average velocity was 2780fps with a 32fps spread on four shots
high was 2792fps
30 degrees / cloudy no wind

yesterday
44g varget 150g ballistic tip (seated at same depth as previous load)
.80" group
average velocity was 2818fps with a 16fps spread on three shots
low was 2814fps
42 degrees / sunny no wind

so obviously I found a sweet spot to work with, but why the velocity change? ambient temperature? a little more fouling in the bore? I did not clean between these two groups.

I was expecting a 30-40 fpd drop... not a raise?
I'm not complaining....... just boggled
wondering what your thoughts are???????
:lol:

 

[Divernhunter]

Weather/temp is my guess if the chrono was set up exactly the same way and distance from the rifle.

 

[chet]

well..... the chrono may have been a foot or so farther from the muzzle yesterday....... I have 12" shoes and I always just step off 10 feet and set it up...........
I'm assuming that the bullet starts decelerating immediately after leaving the muzzle.... no way in hell can it accelerate after exiting, right?

 

[jmad_81]

I had the same thing happen to me with a 270 WSM a few weeks ago. Shot it one weekend with 64 gr. RL-22 and was getting 3254 av. Was a bit warm. Next weekend went out with 63.5 and got the same speed of 3250 and sticky bolt lift. The week before the 63.5 had so signs of being to warm and was 30 fps slower. The only difference that I can think of was about a 25 degree temp difference.

I'm going to wait for it to warm up a bit to be closer to hunting temps before I develope more hunting loads that might see some temps above 80 F.

 

[Divernhunter]

Yes as temp goes up so do pressures. Also loads that are ok at high altitudes may be too hot for lower altitudes

 

[DrMike]

Two quick observations on your report, chet. First, given the reported standard deviation, it would appear that you maxed out on velocity at or before you reached 2818 fps. There is no real difference between the reported velocities given the small sample size from which you generated data. You really need a ten-shot group to draw a conclusion that the velocity differed significantly. I would suspect that your loads are essentially maxed out. Also, the temperatures you report are not sufficiently high to be a concern, even were you "tickling the dragon's tail." It is always a good idea when you are attempting to generate data to begin from the same point (i.e. a clean barrel). However, unless you know the rifle fouls easily, it is not likely that fouling is the cause of the slight differential in velocities. Everything considered, it would appear that you have reached a transient velocity plateau.

 

[woods]

Sometimes the slightest change can make a huge difference

mostly unexplainable but also mostly unduplicatable

 

[7mmXBOLT]

Two quick observations on your report, chet. First, given the reported standard deviation, it would appear that you maxed out on velocity at or before you reached 2818 fps. There is no real difference between the reported velocities given the small sample size from which you generated data. You really need a ten-shot group to draw a conclusion that the velocity differed significantly. I would suspect that your loads are essentially maxed out. Also, the temperatures you report are not sufficiently high to be a concern, even were you "tickling the dragon's tail." It is always a good idea when you are attempting to generate data to begin from the same point (i.e. a clean barrel). However, unless you know the rifle fouls easily, it is not likely that fouling is the cause of the slight differential in velocities. Everything considered, it would appear that you have reached a transient velocity plateau.

Not to mention Varget is Non-hydroscopic and is considered a very stable powder in even the most EXTREME temperature swing conditions. Probably why it is part of Hodgdons Extreme line? But DrMike is spot on with your load telling you your pretty much at the max. I have found with the Varget that 1 to 1.5 grains below max gave me the best SD & ES and generally proved to be most accurate after messing with seating depth a bit. But those are my rifles!

 

[Desert Fox]

As DrMike alluded, there's not enough sample data to form an analysis. 22 fps ES difference between the two shot groups is not that big of a spread. It could be the result of a dirty barrel, a change in environmental conditions, chronograph distance from the muzzle or a combination of all might have contributed to slight increase in velocity.

 

[old #7]

Going from a 4" group to a .8 could ba as simple as too much coffee. :lol:
Some times when a load gets maxed out you will experience a reduction in velocity. I don't know the sceince behind it but, I have experienced it many times.
Sounds like you did end up finding a good load :grin: . So.... why worry about it :wink:

 

[chet]

I'm not worried about it at all. I just love reading what you guys have to say, we're all intelligent with our hobby, and when we all add in our knowledge into one big bucket.... well, that bucket just happens to be the best resource of all. thanks to all!

 

[Greg Nolan]

If you listen to Dr Mike you will never go wrong!
Like the "Chaos theory" (JURASIC PARK) every result is random based on probability and may take a differant course.
Repition is the best guarantee.
DrMike is right in that Varget is a very stable powder. SO What are the likely candidates? Powder, primer, tempreture, seating depth (you said that wasn't a factor.) The distance from the muzzle may make a differance in velolicty but not group size. Where the loads from the same lot of handloads or factory loads. If all else fails I'll give you $10 for your looser rifle. :lol: Seriously, You may have a winner but just need to gather further and more long term data. Best of luck and keep us up on how it goes so we can learn.
Greg

 

[Antelope_Sniper]

2780 to 2818???
A difference of 40 fps is nothing. I have loads I've chronoghaphed over a period of years. Some will vary 200 fps, and shoot the lights out everytime.

 

[chet]

A difference of 40 fps is nothing.

Just to make sure you understand

It went faster with less powder.

Under this circumstance, 40fps IS something.