Some 30 Cal BCs Tested

[bailey1474]

Great info. Thanks a ton for sharing this. Just shows you that most manufacturers are a bit optomistic.

 

[Jon A]

Mike, I just needed to know how the chronos read relative to each other. If one read 20 fps higher than the other and I didn't know it, that would skew the numbers one way or the other depending upon whether I put it in front or back. The relative comparison between bullets would still be valid, but much less absolutely accurate.

As it turned out, Oehlers are good. /ubbthreads/images/graemlins/laugh.gif One averaged 4 fps slower reading the same bullet than the other (accounting for the 1 fps the bullet lost between the two). I only shot three to come up with that, but they were all single digit differences with a max of 7 fps and an average of 4. So I simply added 4 fps to the readings from that one when figuring the BC's to put them on a level playing field. Had the difference been bigger, neglecting to do this would have had a larger effect. But the only way to know for sure was to find the difference in the first place.

 

[jwp475]

Nice Jon thanks for info.............. /ubbthreads/images/graemlins/wink.gif

 

[hoghound]

Excellent work Jon, thanks for the effort. That's why manufactures and bullet makers hesitate to put a number on the BC of a bullet. I wish people would understand the number they are giving is a starting point or the number on that perticular day in that location they tested and don't critisize if they can't come up with a match. BP,humidity,temp,and altitude will certainly give different BC on a bullet. What powder or powders did you use for your testing?

 

[Jon A]

Here were the loads:

173 101 Retumbo
180s (all of them) 99 Retumbo
200 96 Retumbo
210 94 Retumbo
240 101 US869

Some of the velocities were higher than they should be. I was starting out the test with a dirty rifle and didn't realize I had forgotten to bring my cleaning stuff until I had everything set up...so I said screw it and did the test anyway.

This makes a difference for some bullets because my throat is really tight and when there's a bunch of carbon built up in it some of the more pudgy-nosed bullets (240 SMK, 200 AB, etc) chamber with significant feel and marks will be on the bullets when removed. I think this sort of has a similar effect to jamming the bullets in the lands, increasing pressure and velocity.

Anyway, that's why a couple of the velocities are higher than they should be. For example, I chronoed the 240 load the next day with a clean rifle and it was right at 2970 where it should be.