Need help with MV changing from one day to the next.

[Mr.Knowit all]

The last few times I went to the range my MV widely varies. My ES/SD is mostly the same but MV is considerably higher or lower even with H1000. I know temp. can play a large roll but it hasn't been more than 20 degrees warmer or cooler every trip out to the range.

I have 2 loads/examples:

Today:
Berger 230 gr. OTM CCI 250, Norma brass, 75 gr. H1000
MV 2736 fps.
ES 8 fps.
60 degrees outside temp.

Last week:
Same load
MV 2776
ES 11 fps.
51 degrees outside temp.


Today:
208 gr. ELDM, CCI 250, Norma brass, 74 gr. MRP
MV 2872 fps.
ES 17 fps.
60 degrees outside temp.

Last week:
Same load
MV 2933
ES 14 fps.
51 degrees out side temp.

What am I missing? It was actually warmer today but my speeds went down a little...??? I am at a loss. My rifle is clean-no unusual carbon or brass build up and shoots well under 1/2 min. What am I doing wrong and/or do I need to watch for?
Thank you.

 

[jimbires]

what chronograph are you using ? light conditions will change velocity readings on optical chronographs .

 

[284winner]

Even if temps were much higher and lower, H1000 is not considered a temp sensitive powder. It does very well with temp spreads maintaining velocities. The other guys are on to something with your chronograph and false readings. It's the first place to start before you start considering the firearm is the problem.

 

[Mr.Knowit all]

I use a Magneto Speed and it has been dead on over the years-to the best of my knowledge.

I do believe that it is accurate because recently I went out and I was shooting clay pigeons at 609 yards. May first shot was high bc my MV from the trip before was at 2909 fps. and 2909 was entered in my Kestrel. So I hooked up the Magneto Speed and I was shooting 2938 (average with a ES in single digits). I took a .5 min off the DOPE and I hit the rest.

The reason I say that my chrono is probably good is bc what ever speed I am getting that day I will plug it into my Kestrel and it's on the money.

I am at a loss...

 

[Ned Ludd]

Something was clearly different. Both loads were significantly faster by ~40-60 fps on the previous outing. Unfortunately, now you are in a position of deciphering exactly what was different. As other mentioned, it could be the chronograph, but I also use a MagnetoSpeed and that would not be my first guess. How sure are you of the temperature readings? Did you clean/not clean in between range trips? How did you throw the charge weights? Did you load in the same brass prep? Did you use exactly the same lots of powder/bullets/primers, etc., for both trips? There are a number of possibilities that could have caused the differences you observed, but the chances that someone on the internet will correctly guess the exact cause are iffy. I would continue testing and carefully recording your velocities on different days, and see whether the trend continues. Sometimes identifying all the likely variables, isolating them, and testing them one by one is the only way to identify the exact cause. As much as you would like to know the answer, it may simply be easier to determine whether or not your velocity excursions continue. If not, I'm not sure I would spend a great deal of effort trying to figure out why. If they do continue, then you'll have to isolate the causative factor by identifying and individually testing each of the most likely culprits.

 

[jimbires]

virgin brass will lower your velocity .

 

[rimfiremac]

Was your bench rest or shooting position different? I think this has more effect on both velocity and consistency than many folks appreciate

 

[Mr.Knowit all]

virgin brass will lower your velocity .

Hmmmm..... now that maybe something to look into. I never noticed that before.


I shoot from a table when I can.

 

[Ned Ludd]

In my hands, virgin brass does lower MV, but it's usually only a small amount, perhaps 5-8 fps or so in .223 and .308 cases. Presumably, a small amount of energy goes into expanding the case to the chamber wall during the fire forming process. On subsequent firings, the amount of energy taken up by the expansion process is markedly reduced as compared to the first firing. Firing position alone can account for similar velocity discrepancies. There are many possible sources of the velocity change you observed, narrowing down the choices to the actual underlying cause may not be so easy as I stated previously. Much better to see whether it continues before trying to define the cause.

 

[IdahoSharpshooter]

how many rounds on the barrel?

 

[T-shooter]

The last few times I went to the range my MV widely varies. My ES/SD is mostly the same but MV is considerably higher or lower even with H1000. I know temp. can play a large roll but it hasn't been more than 20 degrees warmer or cooler every trip out to the range.

I have 2 loads/examples:

Today:
Berger 230 gr. OTM CCI 250, Norma brass, 75 gr. H1000
MV 2736 fps.
ES 8 fps.
60 degrees outside temp.

Last week:
Same load
MV 2776
ES 11 fps.
51 degrees outside temp.


Today:
208 gr. ELDM, CCI 250, Norma brass, 74 gr. MRP
MV 2872 fps.
ES 17 fps.
60 degrees outside temp.

Last week:
Same load
MV 2933
ES 14 fps.
51 degrees out side temp.

What am I missing? It was actually warmer today but my speeds went down a little...??? I am at a loss. My rifle is clean-no unusual carbon or brass build up and shoots well under 1/2 min. What am I doing wrong and/or do I need to watch for?
Thank you.

Keep track of your barrel and chamber temperature. I use a small Coleman Quick Pump with a piece of hose that runs off a 12v battery box. Just insert it in the open bolt and let the air blow through out the muzzle for 30 to 90 seconds depending on the temperature. 60 seconds cools better than setting 5 minutes with the muzzle pointing straight up. and there is no thermal shock to the metal.

 

[Gottschall]

Are you placing the chrono the same distance from end of barrel every time? Measure with a tape measure and be sure to push the gun back to the same spot after recoil. Jeff

 

[dcali]

I'm going to guess it's the brass forming up to the chamber, and that it will settle out now. Just a guess though.

 

[Canadian bushman]

virgin brass will lower your velocity .

Ive noticed this as well.

One reason i run my chambers at the bottom or below SAAMI spec.

 

[hogpatrol]

Some WAGs, any changes in cleaning regimen? Carbon ring boosting pressure? Different lot of bullets? Fired brass versus new?
Seating depth variations?