Help with BC calculation

[milanuk]

Chessman,

Good to see we have a 'expert' on hand!

I'm not arguing that a G7 B.C. isn't a more accurate model than the standard G1 that almost all (all?) manufacturers give out. And I'm not arguing that the numbers come closer using the 'correct' model for the type of bullet used.

What most people have available to work with are G1 B.C.s, and that's what they 'think' in when comparing one bullet to another. Right, wrong, or otherwise that's my experience. Partly because thats what manufacturers use, partly because some software still doesn't give options for converting a B.C. from one drag function to another, and partly because for most instances, it's 'close enough' for most people.

The B.C. of this bullet may very well be higher than others out there. But being the engineer here, what do you think the probability of a bullet designed and marketed, as I understand it, as a controlled expansion hunting projectile, not a ultra-low drag target bullet, is going to have what, 25-30% better ballistics than the current 'best' offerings out on the market now ('exotic' designs from Lost River or Powder River or whatever excluded in this case)? To me, that falls in the realm of 'possible, but kinda unlikely'. That's all.

Like I said, if it's true... yeowza. If the accuracy was comparable to a MatchKing or Scenar... and that kind of flat shooting B.C.... I for one might have a new LR bullet for my 6.5-08! Might have to try some in my gun just for my own peace of mind after all this discussion anyway.

Let's keep the discussion going, at any rate. I'd love to hear results from anyone else shooting the Hornady 6.5mm 140gr SST bullets.

Monte

[ 08-20-2004: Message edited by: milanuk ]

 

[Guest]

I can see that Hippy Cabbage can still be found in mass quantities in this neck of the world.

Jerry, Jerry, Jerry, no 6.5 bullet known to man, fired at less than 3000 fps. from a gun with a remotely normal scope height, is 8 moa up from 100 to 600 yards.

I would highly suggest that you zero your gun at 100 yards and hold that zero on top of an 80 inch target at 600 yards and measure the actual bullet drop. Your scope clicks prove nothing about trajectory, and could not be correct.

 

[milanuk]

S1, I think Jerry mentioned in one of the early posts in this thread that he had verified the click values for the scope already.

Seems like weird ballistics is contagious... I've got a 6.5-08 that takes a full minute less to get from 300 to 600 than it should (8.25 vs 9.25). Most likely a 'click' issue though in my case.

Monte

 

[Jeff In TX]

No matter what ballistic calculation you use, he has really no mathematical ballistic curve for his results. His bullet begins to descend at very abnormal rates according to his yardage. Its slows at one range and one rate, then the bullet almost stops descending, the it slows again and then between 700 and 800 yards it really starts to die.

The only logical explanation if his velocity was correct, his range markers were really off. Here are two examples. The first with his 140 gr SST bullet at 3200 fps which gave me a close reading to his 300 and 800 yard results. The second is at 2975 and should resemble a more normal ballistic curve for his bullet

Yards * Your results * ballistic program results using a velocity of 3200 fps
300 * 2.5 * 2.41
400 * 4.0 * 4.4
500 * 6.0 * 6.6
600 * 8.0 * 9.2
700 * 11.0 * 12.0
800 * 15.0 * 15.17

If your velocity was really 2975 your actual ranges would more likely be
Yards * Your results
275 * 2.5
345 * 4.0
425 * 6.0
500 * 8.0
600 * 11.0
715 * 15.0
800 * 19.10
900 * 23.01

[ 08-20-2004: Message edited by: Jeff In TX ]

 

[chessman]

First, I apologize to the group if my post last night came off as arrogant. I'm not a ballistics expert and have never claimed to be. I did want to establish some credibility as a physicist. What I do know is high-speed aerodynamics.

Dissect Jerry's original post. He knows the mass of the bullet and muzzle velocity, and he has measured the change in velocity over time by measuring the drop. Now he is looking for the math that describes the arc that the bullet flies – effectively, the equation that describes the aerodynamic drag on that particular bullet fired from that particular barrel. We don't know enough of the variables to make any absolute statements about BC or drag function. We still don't know the scope height, atmospheric conditions, or rate of twist.

The G1 function isn't a bad choice for the tangent ogive bullets of yesteryear. Those bullets all had the same basic shape and produced the same relative drag. The new secant ogive bullets do not have the same drag, so using one drag function for all bullets will not give accurate results for all bullets. If the G7 gives a closer match to the measured results, then why would we not use it for this bullet?

I still say the G7 function is a closer match to the observed results – maybe not perfect, but closer than G1.

If the equation doesn't match the results, then something is wrong with the equation.

 

[Mysticplayer]

Glad to see that everyone has their thinking caps on. Here is some more measurable data to throw at this situation.

Scope height - boreline to mid of scope is 2". Weaver bases and Burris high rings with inserts. Nothing wacky there. Scope is shimmed slight 'nose' down.

Leica 800 rangefinder used to range. Readings it gives are +/- 3 to 5yds so not too bad (within the 1% error I would expect from any device of this type). In absolute measurement, it may be off but I have no way to confirm. At least it is repeatable.

Scope come ups have been compared as have the mil dots at 100yds. The adjustments are accurate to within what I can see and the group size of the rifle. Again, not perfect but we are not shooting lazers here. The rifle is shooting 1/4 to 3/8's MOA at 180yds.

Mechanically, the scope and rangefinder have repeatable results. I range a certain distance and come up on the scope, pull the trigger and there is a hit on a rock. Is there error based on target size, absolutely. Maybe a 4 to 6 inches either way - not the few feet of difference a drop table would suggest.

Another measureable is the length of the bullet. These bullets are a fair bit longer then the MK or Lapuas. So they should have a better aerodynamic shape. Haven't compared to Berger VLD's or Lost river because none available to compare.

All I can say is someone(s) go buy some and shoot them in a 9 twist. If your results are even remotely close to mine, we are definitely onto a good thing.

As far as an absolute BC number, really doesn't matter. Most published BC numbers don't mean much anyways as the assumptions change or the math is flawed. Also, each barrel creates its own results.

As one said, the only way to know is to shoot at paper and measure where the holes show up. I have shot at rocks and the results warrant a much closer look.

And yes, for a hunting bullet, they are very accurate. I have shot these in two rifles and put them in the 1/4MOA range.

Strange things are happening...but that's good.

Jerry

 

[Matt27]

Jerry

I have a 260 with 24" sporter barrel and had to go get some of these and try them. I hopefully will be able to get to the range tomorrow. I can get all the way out to 1000. I won't be able to get 2900fps but should be able to get up to 2800.

I worked up a load with the 139 scenar last week and shot it from 300 all the way out to 1000. The accuracy speed for this bullet in my gun is around 2750fps. I am at an altitude of 4250 and it was 85degrees out and only took me 27min to get to 1000 and had a 10-15mph breeze from 12:30.

Hopefully will be able to report back tomorrow with the 140's. They are definatly longer than the 139's and a shorter bearing surface also(at least to my eye).

 

[Matt27]

Jerry

Off of Hornady's website they say the 140 has a B.C. of .520. Running your numberson infinity with a 180yard zero with a b.c. of .7 i get
800-15.1 moa
700-12moa
600-8.5moa
500-6.6moa
400-4.5moa
300-2moa

With a 100yard zero with a B.C. of .82 i get

800-15moa
700-12.4moa
600-9.7moa
500-7.3moa
400-5moa
300-2.9moa

As one said seems to match the 300 and the 800 moa's but seems to fly flatter in the middle with your come up's.

Interesting and hopefully will report back.

 

[Mysticplayer]

Matt, love to hear how you make out. I also found that 2750fps was a good vel for the 260Rem and 140gr bullets.

I hope that you will be able to get a 100yd zero the same for both bullets then compare the drop out to 1000yds. Measure muzzle vel for both so that a fair comparison can be made.

Good shooting...

Jerry

 

[winmagman]

Jerry
You posted that you were using 58.5 grs of H4831sc in an '06 case, is that correct? The reason I ask is because that's nearly 9 grs. over the listed max in the Hornady manual for the 140 sst. They list their vel. at 2800fps for 49.4 grs of H4831, I would think 58.5 grs would be clipping along at more than 2975 fps.

Just curious
Chris

P.S. I'm curious enough that I ordered 2 boxes to try in my 6.5-284.

 

[Mysticplayer]

Yes, that is the correct load and yes, it is toasty. I know it is not above the vel I have listed simply because of comparisons to other 'sister' cartridges - 25-06 and 270win. Plus my chronograph has been used to measure other rifles and are ok.

The loads listed in the Hornady and Nosler manuals have a much lower pressure then is achieveable. They are using the '06 limits, not the 270/magnum limits.

I am loading to the 270 pressure levels and getting similar vel when comparing relative bullet weights and powder consumption.

The rifle only has a 24" barrel. I am not going to exceed 264 ballistics using a shorter barrel and a lot less powder. Pressures are at 270/magnum not insane levels.

Let us know how your testing turns out.

Jerry