Any one publish a G7 BC for Sierra's newer pointed .224 90gr SMK?
- Thread starter JAlfred
- Start date
skiutah02
Silver $$ Contributor
In here is some relevant (real world) info. Scroll up a few posts after following the link.
http://forum.accurateshooter.com/th...let-match-report.3892402/page-2#post-36813163
http://forum.accurateshooter.com/th...let-match-report.3892402/page-2#post-36813163
Ned Ludd
Silver $$ Contributor
0.288 G7 BC for the SMK90 seems extraordinarily generous to me. In general, if you take the G7 BC of many of the typical bullets we shoot in F-TR, they will range from about 47% to just over 51% average of banded G1 BCs from 3000 fps to 1500 fps. For example, the average G1 BC of the Berger 90 VLD is ~0.576 (Litz, Ballistic Performance of Rifle Bullets, 2015). The 90 VLD G7 BC is reported as 0.273, or ~47.4% of the averaged G1 value. The G7 value reported for the SMK90 in the same book is 0.257, but that is an unpointed value.
In my hands, pointing of the Berger 90s gives me approximately a 5% increase in BC as assessed by known MV and recorded drops at various distances. That means the G7 BC of my pointed 90 VLDs is approximately 0.286. Using that value with my velocity puts me spot on out to 1000 yd, so I believe that estimate is valid.
I can tell you from simply looking at the two bullets that the SMK90 does not have a higher BC that the 90 VLD. It's not a huge difference, but the BC of the SMK90 is most assuredly lower. I have also shot with a number of people using the SMK90s and their windage and drop values from known MVs point to exactly the same conclusion. Using the 5% increase estimate and Bryan's unpointed G7 BC value of 0.257, I would guess the pointed SMK90s would have a BC of around 0.270, which is about 5-6% less than that of the 90 VLD. This may vary somewhat from lot to lot as these values always do, but .270 is likely a reasonable G7 BC estimate for ballistics purposes. Again, we're talking about differences in the two estimates here of about 6%. That won't put you out of the aiming black at 600, or even 1000 yd, the estimated drop differences at those distances being about 2" and 12", respectively, for MV = 2850 fps. However, I always like to have the best estimates I can so that predicted and actual drops are as close as possible.
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I've shot a bunch of them. Good bullet but not .288.
Not disagreeing with you. Just converting the advertised BC from sierra.
Adam,
sorry, but G1 and G7 are two very differently shaped curves. Because of this, the relation between the two cannot really be expressed by a simple, constant conversion factor. At every bullet speed this conversion factor would have to be different. A given factor either is only usable for a very narrow velocity band (means short range) or is only a rough average that in practice has little value.
Manufacturers in my opinon stick to G1 because it makes the BC numbers look higher, not because it is of any value for modern slender bullets.
In other words, one cannot compare the body shape of Sophia Loren with that of Sylvester Stallone by simply using one number.
sorry, but G1 and G7 are two very differently shaped curves. Because of this, the relation between the two cannot really be expressed by a simple, constant conversion factor. At every bullet speed this conversion factor would have to be different. A given factor either is only usable for a very narrow velocity band (means short range) or is only a rough average that in practice has little value.
Manufacturers in my opinon stick to G1 because it makes the BC numbers look higher, not because it is of any value for modern slender bullets.
In other words, one cannot compare the body shape of Sophia Loren with that of Sylvester Stallone by simply using one number.
I wasn't implying that. I was just saying it's not as high as Sierra claims. Like Greg said, you can just look at them and tell the 90 Berger will have a higher BC.
Surprisingly, it is just a simple conversion. Here is an older post that explains it in detail: http://forum.accurateshooter.com/threads/figuring-g7-based-off-g1.3893552/#post-36699032
Ned Ludd
Silver $$ Contributor
Adam is correct, the conversion from averaged banded G1 BCs to G7 is fairly straightforward, and a simple conversion does work for the majority of applications. In his book entitled, "Applied Ballistics for Long Range Shooting, 2nd Ed." the values Bryan Litz published for a number of G1 and G7 BCs were very uniform in that the multiplication factor to go from G1 to G7 was almost uniformly 0.51. In a more recent book entitled, "Ballistic Performance of Rifle Bullets", conversion between the updated BC values exhibit a slightly wider multiplication factor range, from about 0.47 to 0.51, in order to exactly match the experimentally-determined values. If you take the average of banded G1 BCs over 3000-1500 fps, which cover the velocity range we are typically interested in, and divide by 2, you'll end up with a G7 value that should be reasonably close to the true value. None of these estimates are "perfect", but should be close enough to get you on the target, even at longer distances.
Adam's use of that general formula above was valid; however, the G1 (starting) value he used (0.563) was slightly inflated because it is the single banded value (from Sierra) for velocities of 2080 fps and above. If you take the average of Sierra's banded velocities (>2080 fps = 0.563, 1640-2080 fps = 0.550, and <1640 fps = 0.480), you get an averaged G1 BC of 0.531, which when multiplied by 0.51 would give a G7 value of 0.271, very close to what I estimated using Bryan's actual data and the 5% pointing increase fudge factor. I do not know whether Sierra's current BC values represent the fact that they now come pointed from the factory. I suspect that they do represent the pointed form. If so, then my estimate of .270 G7 BC was reasonable. If Sierra's current G1 BC values do not represent the fact that they are pointed, then I think the values are slightly inflated as based on Bryan Litz' testing and the results of people I know that have been using them.
In my hands, if the estimated BCs are within about 3-5% of the true value, the ballistic calculator predictions match very well to what I see in terms of actual drops at various distances using known MVs. In other words, it's not really noticeable above the noise. If the estimates differ by 5-7% (or more), the predicted drops based on a known MV will be off noticeably. As with almost everything, it boils down to whether or not I can shoot the difference (or reliably detect the difference). If the amount by which an estimate differs from a true value is not large enough that I can reliably shoot or at least detect the difference, then the estimate provides a valid and useful working tool for me.
None of that means that I am not willing to update "estimates" as new and pertinent data become available. However, in F-Class we have a pretty large aiming black on the target. All I really need to do is get on with the first shot, then I can correct as necessary based on the shot marker position. Those are not very stringent accuracy requirements, and I can tell you that as long as the wind conditions aren't extremely challenging, I am rarely outside the 7-ring or 8-ring on my first sighting shots at 600-1000 yd using these type of BC estimates. However, making first round hits on a much smaller target certainly requires better estimates than I normally need. If that was my requirement, I would start with the BC estimate, then determine my actual DOPE on multiple days over a range of atmospheric conditions, if possible, to obtain the best possible data with my specific setup.
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We have the new MatchKing Pointed in our library. Including a CDM.
stacy
2 TIMOTHY 2 VERSE 15
NOT ONLY DOES THE BERGERS HAVE HIGHER BC THEY SHOOT BETTER IN MY 1/7 223 ACKLEY
I shot a 200-16x at 600 with the Sierra's so they are capable of decent accuracy as well.
stacy
2 TIMOTHY 2 VERSE 15
GOOD DEAL I MAY WORK WITH A LITTLE MORE GOT SOME POINTED 90GR NOW
Shot a 200-13 with them just the other day. won the club match with a 597-33 for 500 yards. I wouldn't be afraid to use these in comp anywhere up to 600 yards. I should also mention that not one came apart in flight. Now I wish I could say the same for the VLD 90.
Have you had the vlds come apart in flight or did I read that wrong?
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