Interesting Observation

[rduckwor]

Interesting observation. 308, Remington 5R with about 1200 rounds down the pipe. I went out to shoot a short OCW work-up with some new primers and Berger 175Gr OTM bullets. Noticed that the velocity was progressively lower as I work thru the second and third round of each charge weight. Lapua brass, Wolf primers, brass matched as to number of firings, date of processing, etc., Varget powder.

I shot these round robin (Low to high, high to low, low to high by charge weight) And did not allow for barrel cooling in between each round. Note that the fourth charge weight (44.9 Gr) did not exhibit the progressive drop in velocity across the spread of rounds.

Berger 175 OTM
LOAD 1 2 3
44 2664 2643 2619
44.3 2662 2646 2625
44.6 2670 2659 2644
44.9 2689 2690 2675

I puzzled about it and was about to write it off as an issue with the scale that I trickled up on, but then remembered that I also shot an OCW run with SMK 175's which did not exhibit this decrease in velocity. Shot that run in the same manner after a brief barrel cooling period.

SMK 175

LOAD 1 2 3
44 2623 2640 2631
44.3 2662 2661 2648
44.6 2665 2646 2660
44.9 2674 2663 2680





What's going on with the Bergers that I need to know about?

Thanks,

RMD

 

[Warren Dean]

Looks like the 2690 is an outlier. Is it repeatable?

I wouldn't place too much emphasis on such a small sample.

 

[memilanuk]

Something I've seen in the past was that the muzzle velocities out of a cold clean barrel were different from a hot, dirty barrel. The initial velocity was often higher initially and dropped slightly as the barrel fouled a bit and it was no longer bare bullet jacket riding against the lands.

To me it looks like the numbers from the SMKs, which were "shot after a brief cooling period" were from a bore that had stabilized in terms of fouling, temperature, as opposed to the numbers from the Bergers, which look like they came from a cold clean barrel. If the barrel was in fact warmed up and fouled when you shot the OCW with the Berger OTMs... then we're back to instrumental error and small sample sizes being unreliable indicators of much of anything.

 

[rduckwor]

The barrel was fouled and cold initially when I shot the Bergers. As I said, I shot them round robin, so after the initial cold bore shot, there was no bias toward any given charge.

I was shooting over a Shooting Chrony, admittedly, not a precision instrument, but stable and reproducible in my hands and experience.

I guess I will simply have to load and shoot more of the Bergers for statistical purposes and see what I see.

Thanks,

RMD

 

[memilanuk]

Take a look at this plot of the velocities vs. shot #.

Hopefully this illustrates what I am trying to say... see how the velocities all follow the same trend... and the shots for each 'string' (1-4, 5-8, 9-12) march up, then drop back down (next 'rung' or increment in the ladder). If we ignore the shots for the 44.9 load, the velocities march up for each charge increment, and then drop back down like you'd expect, but the 44.6 velocity is less than the 44.0 for the previous 'string'.

Look at it in this view, where I reversed the shot order within each 4 shot 'string'. Do you see what I'm talking about? Do you see the consistent linear progression to the velocities? Not sure what is causing it, but I'm going to go out on a limb and say I seriously doubt it's the bullets...

 

[rduckwor]

Mem:

I do understand what you are saying. However, I can't explain the cause. Is it a statistical phenomenon and a real physial change in velocity?

Thanks,

RMD

 

[memilanuk]

RMD,

There can be a variety of things... the barrel fouling, or just heating up, or lighting changes during the string with many chronos - though the latter more usually manifests (in my experience) as a sudden change in velocities as the sky clouded up.

If'n it were me, I'd load up 10 or so fouler rounds to shoot at the start of the session (we all need more trigger time, right?) and then repeat the test and see if they exhibit the same behavior.

Monte

 

[Spin Drift]

Take a look at this plot of the velocities vs. shot #.

ooooh, I've never seen this before.
I like nerdy math tools.

Thanks.

 

[rduckwor]

Roger the fouler run.

I will try to get that done and report the outcome.

Will be a while as I am up to my ears right now.

As for the light changing, I have seen this happen, but the shooting day was clear and cloudless and covered only about 30" duration.

Interesting. Thanks for all of your thoughts.

RMD

 

[memilanuk]

ooooh, I've never seen this before.
I like nerdy math tools.

I like the ease of posting something that everyone else can easily access the original raw data, and in the long run, I'm hoping if other people start using it, eventually people will start using it to collaborate and moving things forward a bit more quickly.

 

[mshelton]

I work up loads for a local guy that makes smokeless muzzloaders and I've noticed a similar pattern, our velocity will drop off if we shoot over 3 consecutive shots, seems when the barrel starts to get warm it loses velocity. This goes against a lot of what happens with most of our centerfire rifles in that as they get hot they speed up.

Granted it's somewhat of a different ball game with the muzzleloaders, we're using a .451 jacketed bullet that has been swaged to the bore instead of a regular metallic cartridge. Which brings up a thought, do you know what size your bore is, could be that a 300/308 bore will slow down as it heats and a tighter bore will speed up.

 

[rduckwor]

mshelton: I think it has something to do with the difference between bullet diameters and bearing surfaces.

Continuing to investigate.

RMD