Odd velocity readings

[42769vette]

Caliber 28 Nosler
Purpose, 600 yard hunting rifle
Powder 81.9 gr H1000
Bullet 162gr ELDX
Brass Nosler
Chrono Magneto speed
Barrel Bartlein 26 inch 8.5 twist


So I'm having some odd readings, that I'm hoping someone can explain. I have had the same basic results a few times in a row now. When loading a 6 shot batch, I have a velocity jump. velocities on the last group were as follows.

Shot 1=3200
Shot 2=3198
Shot 3-3204
Shot 4=3230
Shot 5=3226
Shot 6=3232


In all 3 batch's I had a velocity jump on either side of shot 3, but the SD's were consistent. Since Ive never heard of a barrel heating up after a couple rounds, and velocity changing this drastically, I'm thinking my Chrono is shifting a little, and my real velocities are pretty constant. I'm trying a new method of load development, so I havent put rounds on target so see what it looks like on paper.

What are the thoughts of the group?

 

[42769vette]

I just had another thought. Im loading on a old acculab 123. Could my scale be jumping? I would think this is unlikley, but possible??? I think all rounds were fired in order of loading, but Im not 100% certain.

I also realize this difrence will matter .1 MIL at 800 yards, so its not the end of the world, I'm just interested.

 

[Aimsmall55]

You’ve got a max spread of 34 fps out of a rifle shooting almost 82 grains of powder. Personally I wouldn’t sweat it at all.
But it is interesting that you’ve got a mad spread of 6 fps on shots 1-3 and then 6 fps on shots 4-6. Personally, I’d plug the average in on a ballistic calculator and let it ride. If your numbers are connecting to target then don’t worry bout it.

 

[Supersubes]

On the good side, you’re never gonna ge three shots at game...


Are you cleaning frequently? If you’re starting from a cold clean bore, I’d expect to see a progression like that if you were.

 

[42769vette]

The barrel has about 40 rounds through it (just finished the build last week). I cleaned it after 10 mild shots, and haven't cleaned it since. I'm not worried about "I missed that shot because of 20 FPS velocity", its more of an interesting thing Ive never seen that I was curious if could be explained.

 

[42769vette]

There was a very good posting at Accurate Shooter on chronograph accuracy and Laurie Holland posted that the amount of sunlight and clouds effect chronograph readings.

He recomended using a magnetospeed chronograph for more accurate and uniform readings.

NOTE, Laurie Holland is a noted British competitive shooter and writes articles for Target Shooter Magazine.

I have had in the past consistent reading in a shot string that were higher, or lower than I expected, but the SD's were consistent. I could see that being light, or cloud's effecting the chrono's reading. These 6 shots are being taken in the course of 2 minutes.


For grin's I just loaded up 5 more same loads, and got

Shot 1=3189
Shot 2=3214
Shot 3=3202
Shot 4=3223
Shot 5=3241

 

[b6graham]

What are you using for a powder scale

 

[42769vette]

What are you using for a powder scale

Acculab 123.

 

[ShtrRdy]

Uncle Ed -- the OP is using a magnetospeed.

 

[Kopfjager1]

If you think your scale could be drifting while throwing your charges, first, let it warm up for 30 minutes prior to using it. Second, Im sure you load your rounds in numerical order, so try shooting your last round loaded first, and work backwards. That will rule out the possibility of scale being at fault.

 

[Diver160651]

Is you barrel clean when you start and several rounds in the speed increases? If so, that's common..

 

[42769vette]

Is you barrel clean when you start and several rounds in the speed increases? If so, that's common..

Barrel is not clean, but the rifle only has about 40 rounds through it, so its not dirty either.

 

[42769vette]

If you think your scale could be drifting while throwing your charges, first, let it warm up for 30 minutes prior to using it. Second, Im sure you load your rounds in numerical order, so try shooting your last round loaded first, and work backwards. That will rule out the possibility of scale being at fault.

My scale stays on all the time.

 

[Deleted member 10043]

You don't have enough rounds down that barrel yet.

 

[Diver160651]

Anytime you start with a clean dry barrel the speeds are slower.

A new clean barrel even worse.

However, even a seasoned barrel can be triple digits slower in the first few rounds and speeds up with fouling rounds.

 

[Milo 2.5]

I have had in the past consistent reading in a shot string that were higher, or lower than I expected, but the SD's were consistent. I could see that being light, or cloud's effecting the chrono's reading. These 6 shots are being taken in the course of 2 minutes.


For grin's I just loaded up 5 more same loads, and got

Shot 1=3189
Shot 2=3214
Shot 3=3202
Shot 4=3223
Shot 5=3241

Your questions were answered on accurate shooter, barrel-chamber heat. You forgot that tidbit on this post.

 

[42769vette]

Your questions were answered on accurate shooter, barrel-chamber heat. You forgot that tidbit on this post.

The barrel heating up sounds like the most likely cause to me. I didn't forget that tidit, I am getting all the opinions people have to offer.

 

[Milo 2.5]

The barrel heating up sounds like the most likely cause to me. I didn't forget that tidit, I am getting all the opinions people have to offer.

I was referring to your barrel contour, if you had a heavy varmint barrel you could probably get a 5 shot string before things went awry. Also, like mentioned, you need to let your barrel get through it's speed up or break in period. Also the amount of spread could be attributed to the light for case bullet, if you were shooting a 195gr, your spreads would not be so extreme, if indeed is it extreme in your view.
As for now, limit the chrono sessions to 3 shot strings, with a cool down between.

 

[42769vette]

I was referring to your barrel contour, if you had a heavy varmint barrel you could probably get a 5 shot string before things went awry. Also, like mentioned, you need to let your barrel get through it's speed up or break in period. Also the amount of spread could be attributed to the light for case bullet, if you were shooting a 195gr, your spreads would not be so extreme, if indeed is it extreme in your view.
As for now, limit the chrono sessions to 3 shot strings, with a cool down between.

Thankyou sir.