Need Help: Velocity Issues

[Bmars06]

Howdy,

I'm having some problems with velocity increases. First, I am shooting a Browning 300 Win Mag X-Bolt. Shooting was done at the same place, same altitude both days. The only difference is the temperature. As you will see, the second round of reloads, using the exact same components and measurements, shoot significantly faster (in some loads, up to 200 fps faster than day one loads). Can anyone help me with why this is happening? I understand that the warmer day can have an effect on velocity, but this much of a difference?

A consensus on the forum seems to suggest that it could either be (1) a change in temperature with senstive powders that are near max loads AND/OR (2) a light differentiation with the chronograph. If either of these are true, how should I address the loads that I have, if at all?

If Heat: Do I need to make different loads for different temperatures or just back of the loads as they are and shoot them at all temperatures?

If Chrono and Not Heat: How do I figure out which more of an accurate reading between Day One and Day Two?

If Heat AND Chrono:.... well.... wtf?

DAY ONE

My first loads were shot on a day when it was about 60 degrees outside and I waited on average 2-3 minutes between shots. The chronograph results are below. Shots listed are in order in which I shot them that day.

DAY TWO

My second round of loads were shot on a day when it was about 85 degrees outside and I wanted on average 2-3 minutes between shots as well. The chronograph results are below. Shots listed are in order in which I shot them that day.


Load 1:71gr (Max suggested charge is 72.3gr) WXR pushing a 200gr SMK

Day One Shots

2826
2824

Day Two Shots This load seems to be within max velocity (which is good, leading me to believe it may be more of a heat issue, not a chrono issue)

2794
2803
2831
2939
2915



Load Two Shots: 75gr (max charge) IMR 7828SSC pushing a 200gr Sierra Spitzer

Day One Shots

2946
2958

Day Two Shots

3190
3176
3174
3211
3176



Load 3: 71gr (Max suggest charge is 70.7gr) H4831 pushing a 180gr SMK

Day One Shots

2881
2895

Day Two Shots

3023
3019
3022
3002
3013



Load 4: 77gr (Max suggested charge is 76.8gr) IMR 7828SSC pushing a 180gr Spitzer

Day One Shots

3091
3068

Day Two Shots

3315
error
3295
3292
3299



Load 5: 74gr (Max suggested charge is 75.5gr) H4831 pushing a 168gr Hornday A-Max Moly

Day One Shots

3053
3069

Day Two Shots

3130
3126
error
3115
3176



Overall Question: Why is there such a big difference in velocity between the two different days of shooting? Also, please see the questions at the top pertaining to the various potential issues.

Thanks for your help!

[knucklehead0202]

temperature will do it except when using certain powders. if you have a pretty hot load in the winter, you may not want to shoot it in the summer. if you're at or close to a max load, you'll see differences, sometimes major when the temp is higher. inspect your fired cases, and compare if you still have some from both days of shooting, that may tell you something.

[Ferrum]

Day to day the loads are pretty consistent...

My guess is that the difference is not in temperature but the change in light from one day to the next on your chronograph.

Found this on one of the manufacture's web sites FAQ's...

"Any chronograph that relies on sunlight to achieve a velocity reading is governed by the amount of sunlight that is received by the unit. Since sunlight is an electromagnetic radiation2 (EMR) source of Visible, Infrared (IR) and Ultraviolet light (UV, among others, that changes with atmosphere concentration and density as well as it’s angle (time of day). Variables in its power and composition exist thus affecting the read out of these chronographs. To put it simply, if your chronograph relies on sunlight, expect variable results from day to day and sorry to say hour to hour."

[chad allred]

Its temp dude...
I loaded some sierra 45 grain round nose for my AR with a 1-7 twist...shot 1.5" at 100 yards at 70 degrees

Shot the same batch at 85 degrees and they blow up at about 50 yards...just a white "poof" lol
Don't remember the load offhand... its written down at home.

[Bill Steele]

A friend of mine likes a particularly temp sensitive ball powder, he has three different loads, one for winter temps, one for spring/fall temps and one for summer temps.

Looks like the WXR is pretty stable temp wise, like the extreme powders. Most are more temp sensitive than your WXR. You don't suppose WXR stands for Winchester Extreme Rifle, do you..

[Bmars06]

Yea I'm hoping it's just a temperature problem and if so whether I need to back off my loads since they get so much faster...

[Bill Steele]

Yea I'm hoping it's just a temperature problem and if so whether I need to back off my loads since they get so much faster...

How was the bolt lift on Load #4 Day 2?

How did the primers look?

[chad allred]

reread post five...85 yards was a typo...meant 85 degrees!

[Bmars06]

Also, is the major difference in the FPS between certain loads something to worry about? Some loads the second day shots well exceed the manufacturers max listed FPS with the suggested max powder...

[Bmars06]

How was the bolt lift on Load #4 Day 2?

How did the primers look?

Bolt lift seemed normal and primers looked normal... Flush with the case and a clean pin punch. I imagine if the pressures were crazy high it would be difficult getting the case out?

The part that concernse is the 200 FPS differentiation in one of the loads. The WXR seems fairly stable.

Also, how much can barrel temp effect velocity... From a cold barrel to a hot barrel?

[Bill Steele]

Bolt lift seemed normal and primers looked normal... Flush with the case and a clean pin punch. I imagine if the pressures were crazy high it would be difficult getting the case out?

The part that concernse is the 200 FPS differentiation in one of the loads. The WXR seems fairly stable.

Also, how much can barrel temp effect velocity... From a cold barrel to a hot barrel?

It sounds like your pressures were under control. I have not seen much difference in velocities cold to warm bore wise. If you have a clean and shiny bore, the first few shots will usually record slow until the fouling speeds things up, for a time. I always shoot some fouling rounds on a clean barrel before I settle down to measure things.

I am surprised you didn't see any signs of pressure on the hot number 4 rounds. That is almost 7% above the max book velocity which is a lot more than 7% added pressure.

What barrel length are you shooting, that can add some velocity over book loads if they tested on a shorter barrel.

[Bmars06]

26 inch barrel.

Can a change in sunlight or outside temp really change velocities by up to 200 FPS?

How the heck are you supposed to get an accurate reading?

[BLR81]

26 inch barrel.

Can a change in sunlight or outside temp really change velocities by up to 200 FPS?

How the heck are you supposed to get an accurate reading?

Some people stick a led lamp under the shades to maintain a consistent light source. Chrono reads will vary with the source light.

[Divernhunter]

One thing I noticed is that the day 1 shots are 2 and the day 2 shots are more. Two shots are not enough to get a proper sample of readings. Also FPS can change as the barrel warms up, different moisture in the air etc. I shoot a min of 5 shots for any info and 10 are even better.

[Bmars06]

So with all this being said, what should I do in regards to the different loads?

Load 1 seems to be adequate and not a big difference between the two days (which suggests to me that it's not a chrono issue, but more of a heat issue), but the other 4 loads exceeded the day 1 velocity by between 100-200 fps. Obviously the temperatures were considerably different between the two days, but then again, it could just be a chronograph issue.

So do I just stick with the load the way they are and be cautious when shooting on warmer days... or just assume its a chronograph light differentiation and just shoot them as they are regardless of temperature?

Maybe back down the charge a grain or two?