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powder charge vs velocity

 
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  #1  
Old 05-22-2012, 06:52 AM
BML BML is offline
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powder charge vs velocity

Logically one would think the higher the powder charge, the higher the velocity would be right? After doing some more testing last night I have found that's not the case all the time and I'm not exactly sure why.

Here's my ingredients: 6.5x284-H4831SC-Fed 210-Norma brass-AMAX 140g

Here's my numbers from yesterday.

50.0g shot an average of 2983
50.5g shot an average of 3039
51.0g shot an average of 2861
51.5g shot an average of 2879

Why such a drop in velocities above 50.5 grains of powder? Is it because there's more powder, which takes longer to burn, which in turn will affect velocity?

51.0g shot the best group at .358 but if I can achieve similar results with 50.5 I will drop down to achieve that speed. I will always take accuracy over speed but I do like to test other loads thoroughly until I can prove they will or won't work.
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Old 05-22-2012, 10:44 AM
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Re: powder charge vs velocity

there are many reasons why this can occur...
  • statistically insignificant sample size
  • variable lighting affecting chrony
  • bore condition, clean vs fouled
  • barrel temp, ambient temp, ammo temp, powder temp sensitivity
  • shooting position and inconsistent hold from shoulder fired weapon
  • brass volume consistency
  • seating depth
If you can't come up with anything logical, then see if you get repeatable results by alternating shots from each load. If after 30 samples each, the mean of one load is 2x SD apart, then you can have 95% confidence that the MV going forward will be about that much different.

Either way... how did the groups compare? How did POI compare?

-- richard
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Old 05-22-2012, 11:00 AM
BML BML is offline
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Re: powder charge vs velocity

Point of impact was nearly identical on all 4 groups. 50.0g and 50.5g shot .9 and .8 respectively. It was hot outside so I waited 8-10 minutes between shots to allow the barrel to cool. All my ammo was kept in my bag and in the shade until I was ready to shoot so it all should have been the same temperature when fired. I did get very slight pressure signs with all but the 50.0g. The sun was setting the later I got into the testing so I suppose that could have affected the chrono readings some.
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Old 05-22-2012, 12:38 PM
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Re: powder charge vs velocity

I don't have a clue what the book max load states for that bullet and powder but what I can tell you is that you have reached max load at 50.5 for your barrel/bullet/powder combo. This is the main reason to use a chrony when working up loads. You are seeing lower velocity in the two higher powder charges because of inconsistent powder burn. I had the same thing happen with my 7mag and H-1000 in a 24" tube. Book max charge was 70 grains, but I started getting incomplete powder burn after 68.0. All charges from 68.5 to 71.5 hit within .5moa in my 300 yard ladder test but the load I settled on was 67.6. When getting incomplete powder burn, you will almost always see your E.S. numbers all over the place and unexplained fliers. If the 50 or 50.5 had the same point of impact on the target and decent group sizes, I would play with seating depth and try to find a load with either one of them. Hope that helped.
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Old 05-22-2012, 01:10 PM
BML BML is offline
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Re: powder charge vs velocity

Quote:
Originally Posted by jsthntn247 View Post
I don't have a clue what the book max load states for that bullet and powder but what I can tell you is that you have reached max load at 50.5 for your barrel/bullet/powder combo. This is the main reason to use a chrony when working up loads. You are seeing lower velocity in the two higher powder charges because of inconsistent powder burn. I had the same thing happen with my 7mag and H-1000 in a 24" tube. Book max charge was 70 grains, but I started getting incomplete powder burn after 68.0. All charges from 68.5 to 71.5 hit within .5moa in my 300 yard ladder test but the load I settled on was 67.6. When getting incomplete powder burn, you will almost always see your E.S. numbers all over the place and unexplained fliers. If the 50 or 50.5 had the same point of impact on the target and decent group sizes, I would play with seating depth and try to find a load with either one of them. Hope that helped.
It does help, thank you. Both 50 and 50.5 had groups with 2 touching and one flyer roughly .75" away.

Would switching to a magnum primer solve an inconsistent powder burn issue?
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  #6  
Old 05-22-2012, 03:33 PM
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Re: powder charge vs velocity

I wouldn't try a magnum with only 50 grains of powder. I would try other types of Large Rifle primers though. Inconsistent burn mainly has to do with barrel length and burn rate of the powder you are using. Play with the seating depth first then change primers if that doesn't work. Hopefully the seating depth change will take the flier out of the equation. I seat amax's touching the lands usually.
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  #7  
Old 05-22-2012, 04:11 PM
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Re: powder charge vs velocity

you have passed the optimum expantion ratio for that load for your rifle, it is that simple.
case capacity plus barrel length plus caliber = expantion ratio
once you pass your optimum "er" all you are doing is blowing unburnt powder out your muzzle
go back to your fastest velosity, if the preasure is good and the accuracy is good then you have a good load, if not drop that powder/bullet weight combo and try something else
the best thing about reloading is to be able to find and duplacate a load combo that outsurpasses factory loads,
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