Huge Muzzle Velocity spreads

[Scottish Mag]

I have been loading for about 15 years, but I have just come back form the range frustrated at the results of my loads. The issue is a huge variance in MV.

I confirmed each powder charge on a balance scale, and all are 69.5 grains H1000. I am shooting Hornady Brass, Fed 215M primers, and COAL of 3.40 for a 7 RM.

Here are the results. These 7 were fired approximately 3 minutes apart. I probably should have fired 3, then let sit for 10 minutes to cool. my gut feel is either my charges have enough variance that it is impacting MV, or as the barrel heats and expands, I get increased pressure/velocity?
2867- Cold Bore
2996
3122
3152
3182
3108
3152
I was surprised to see MV above 3100 with this load. I have a 25" Brux Sendero contour barrel with a 1 inch brake.

After a 15 minute cool down, the next 3 shot at:
2976
2969
2982
Which is what I was expecting.

Does anyone have experience with these kinds of ES on loads? How did you resolve? I am confident the charges were all identical.

 

[Greyfox]

Were you doing your testing on a sunny day? Assuming that your chronograph is functioning properly, aligned, stable, and at least 15ft from the barrel, your first set of results look like variation caused by lighting. Shadowing, and or reflections off the bullet can easily cause the kind of variations in your first set of data. Depending on your particular rifle, initial shots from a cold or clean bore can also show variation but in my experience to a much lesser extent. Either shooting in a shaded area, or on an overcast day will generally improve readingis. Results can also be improved by constructing shades that stop stay light or shadows from interfering with the optical readers or causing refections off the bullet. A while back I got a Magnetspeed which eliminated the lighting issue, produced accurate results, and made set-up a whole lot easier. iMHO.

 

[Scottish Mag]

Thanks for your note. It was a sunny day. I did have the light diffusers up top, but sounds like I should go in and do a second round of testing on a overcast day.

Thanks much!

 

[4bycamper]

Large SD and higher velocity than expected? Hmmmm

With my 7 saum, I have 7 reloads on the brass. I have learned to clean the inside of the neck with a brass brush spinning in the cordless drill. Then throw the brass in the vibrating cleaner. I also use a silicon bullet lube on the brass when I FL size. This seems to help.

It seems the brass neck will glue itself to the bullet in a dirty brass reload in as little as 3 months on the shelf. This may cause uneven bullet release and higher pressures. I learned this when I had to pull some bullets to change the load. I don't know if the problem is powder residue or brass to copper bullet seizing. But they were tough to pull even with a collet style puller!

Are there any other solutions out there?

 

[LouBoyd]

Does anyone have experience with these kinds of ES on loads? How did you resolve?.

When I shoot though a chronograph I'm also carefully aiming at a target at a distance I expect to use the cartridges. For a 7mm RM that could be 500 to 1000 yards. Besides reading the velocity of each shot I also record the point of impact of each shot. That will immediately show if the problem is with the chronograph reading or if the bullet velocity really is variable. It also shows up characteristics of the rifle - cartridge - bullet combination which a chronograph alone does not detect. Loading for low ES & SD is only part of achieving good accuracy. Adding a downrange target insures that loads work well and make sorting out anomalies easier.

From the information given I don't know what the problem is. First guess would be the muzzle to first chronograph screen spacing, which wasn't mentioned. The bullet model and distance from the lands wasn't mentioned either.

 

[Mikecr]

Your readings seem (to me) to indicate the barrel stabilized to expected MVs/ES. Right?
Did the barrel come out of air conditioning to the bench?
When it's hot out, I set the gun outside (in a case) for a couple hours to acclimate, before going to the range or in the field hunting.

And I must say it, 4bycamper's notions are exactly opposite of reality in my experience.
There is no problem with fouling left in necks, it does not contribute to increased ES.

 

[8andbait]

probably the sun. If possible I try to get the shadows from the diffusers to cover the lenses. If not, wait for a cloud, I typically see lower than expected numbers in the sun rather than higher. Cloudy days typically show close to correct speeds.

 

[Scottish Mag]

Louboyd,
What distance do you use? I had the chrono set up at 12'. Probably should have been 15?