MV question

[243BR]

I finally got a chance to try out my "new" lab radar yesterday. Here's my question, I shot 2 different guns, both 6BR, both Kreiger 1/8 barrels, both Varget (various charges), both similar round counts, both 108 Bergers jammed 0.005. The only difference was seating depth. One was seated 0.029 longer. The longer seating depth showed an average MV 47 FPS slower than the shorter depth. Perhaps a silly question, but is that much difference in MV "normal" with 2 similar barrels?

 

[LA50SHOOTER]

Not Surprising - Each barrel is "Individual" unto its own characteristics. - I as well buy more than one barrel with the same twist at the same length & contour from the same mfgr. and have experienced close to the same results with the exact same load being used. - In your case you had one load seated 0.029 longer which in effect alters the numbers in expansion ratio - even though only slight, enough coupled with the fact that no two barrels are exactly the same.

 

[Bart B.]

We're both barrels' chambers made with the same rough and finish reamers?

 

[dmoran]

With identical charges, the one with bullets seated deeper in the case, would make more pressure, hence more velocity. If the barrels were chambered with different reamers, again there could be pressure/velocity indifference. And of course, no two barrels are identical.
My 2-Cents
Donovan

 

[243BR]

Thanks guys. They were cut with 2 different reamers. I suspected it could be seating depth/pressure. The average difference of 47 FPS was very consistent over the "various charges". I probably wasn't clear on "various charges". They were from 30.0 to 30.9 Varget and were shot in both barrels.
Thanks again

 

[dkhunt14]

Thanks guys. They were cut with 2 different reamers. I suspected it could be seating depth/pressure. The average difference of 47 FPS was very consistent over the "various charges". I probably wasn't clear on "various charges". They were from 30.0 to 30.9 Varget and were shot in both barrels.
Thanks again

I sort of figured you shot both guns with various charges and compared. That was sort of a logical thing to think. It could be just different barrels. I have had Dashers where you couldn't get 2950 and some where you could get 3100. These barrels were cut with the same reamer, same lot of bullets and same seating depth. The only difference was a different barrel. You never know if it will be a fast barrel or a slow one. Matt

 

[normmatzen]

I have played a lot with QuickLoad and found that 0.010 difference in seating depth can cause 20-30 fps difference in Mv. Don't forget, when you play with different jumps, you are actually changing the case volume . So, lesser jump makes for lower Mv.

I have never quite believed "some barrels just like a certain jump." Instead of testing for best shooting after doing a ladder test by finding the best jump, I do a 5 shot group, round robin with 5 or 6 different powder charges and graph the results with Excel and do a polynomial curve fit to find the actual curve of the load Vs Es data. I have found this has indeed found the correct load and changing jump deteriorates the accuracy relative to the ideal load by math. Then, I tune the cantilever resonance with the tuner.