#### Nighthawk

##### Well-Known Member

Thanks Darryl.

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- Thread starter cdixon
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Thanks Darryl.

Can not explain this but am not complaining either.

VH

Could someone tell me the legnth of the 240 gr MK?

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A nice concern to have...I just hope that hornady starts making heavier Amax in 30 cal.

My new project is a 6.5 and I will be using 140gr SST bullets. These bullets measure just under 1 1/2" long. Compared to other 6.5, 7mm and 30 bullets, only the 240gr MK is longer. The BC of these bullets is going to be very high. I expect that it will be higher then 0.625. Same length as the 7mm 162gr SST which are listed at 0.625.

Jerry

135gr sierra match I have listed at .435bc, which seems to be about right for drop at the velocity I have been shooting out to 500yds.

I just was given a couple boxes of 135gr sst's and was wondering if anyone has a BC for those bullets in .277caliber. I'll be pushing them around 2900fps.

What are some of the higher bc bullets in the .277 catagory?

135 Gr Sierra MK----.488

140 gr Sierra GK----.479

140 Gr Hornady SP---.486

150 Gr Sierra GK----.497

I don't have the one you asked about though.

All the above BCs listed are from the Oehler program.

Later

DC

[ 08-24-2003: Message edited by: Darryl Cassel ]

I don't quite believe everything I read, or everything that comes out of a computer.

Thanks Darryl, appreciate it.

I have found that, with any listed BC, it's best to "actual fire" to a WELL known yardage and check the drops for "your" rifle as you have been doing.

You can then go back to the ballistic program and find the BC that the bullet is producing in your rifle and during the weather conditions and tempertures you fired in.

Some BCs do better and some do worse then what is listed.

Fire for effect.

Later

DC

Boat tail bullets use the G5 drag model. The G5 drag model and the mathematical calculations were developed specifically for boat tail bullets, but they produce a lower BC numbers because of the mathematical calculations involved. VLD (Very Low drag) bullets use the G7 drag model and flat base spire point bullets use the G6 drag model.

Bullet manufactures advertise the G1 drag model because of the higher BC number. Higher BC’s sell more bullets. The Sierra .308 168 gr, match king has an advertised G1 BC of .462. The actual G5 drag BC is .289. This is the drag model the US Army uses at the Aberdeen Proving grounds for its calculations. Why, because it is very accurate.

Most of the ballistic programs on the market use the G1 drag model and out past 450 yards the accuracy starts to slide down hill quickly. Some of the better programs such as Ohler still use the G1 drag model, but have massaged the mathematical calculations for better accuracy.

My ballistic program is near dead on to 800 yards for my .308. That is if I input all of the correct data. Beyond that it’s a matter of the me or the ballistic program as to what’s really correct, but were not off by much.

I may have missed the article and I'm not sure what drag curve Rick was refering to when he said the Scirocco was near a 1.0 BC but, if it was on the G1 curve.... he's all wet to say the least. I have the Oehler 43 data that says it's .560 - .585 BC, measured at 300 yards with an acoustic target. This is the most accurate BC I've gotten from this bullet using the 43 PBL.

The problem with measuring BC's with the Oehler 43 and acoustic target, which Rick probably used in his tests, is that it calculates the downrange velocity based on time of flight, and with that it calculates the BC.

The problem with TOF measurments is if the inputed distance to the target was off by more than a tenth of a foot or so, it simply gives meaningless data. Even using a steel tape and very carefull measuring, it's possible to be off a tenth or two in the distance required to get accurate BC numbers.

The better way is with two seperate chronograph rails calibrated together near the muzzle, then with one set down range and hooked to the 43 or a seperate 35P unit... or get a dead on zero at 100yds then measure the "actual" drop in inches at a given distance beyond that while recording MV on a "good" chronograph. Take this MV and drop info to a ballistic program and use the average MV and enviromental data you collected, then manipulate the BC number until the actual drop on the ballistic chart matches what you were getting at the range... that's the most accurate way.

If you used two chronographs to test, look for the downrange velocity to match what you were getting as you manipulate the BC number. Ideally the drop in inches and the downrange velocity should point you to the same BC and testing both at the same time would be best.

The BC that I remember matching the 180gr Scirocco, and also the 180 Ballistic Silvertip at 3200 fps and my actual "scope settings" of 22.5 moa at 1000 yards and 7.7 MOA at 500 yds was between .530 and .550, nowhere near 1.0...

Barnes bullets are the only ones that I've tested that produce quite a bit different BC's than advertised, and most are much lower in reality. The 168gr XLC was an exception and was real close to advertised.

Like Jeff said, once you get out past 500 - 600 yards, the G1 curve may not be the best drag curve to use, and probably wasn't to begin with, but it will work to a point. Using it might even give you a different BC than the chrono downrange did. Why? The G1 drag curve not matching your actual trajectory curve. If the middle ranges don't match the trajectory curve your MV and new found accurate BC indicate, you must use a G5 or a G7 curve, which ever fits your "actual" fired drops best. Do yourself a favor and fire at least a five shot group at each distance and use the center of each group for the referance data point to match the curve to, a single shot at each 100 yard distance or whatever you decide on tells you absolutely nothing.

Jeff,

It's my understanding that a 1" artillery round with a 1.0 BC was the referance standard for the G1 curve, not a 45 ACP?

Oehler's Ballistic Explorer program does give the option to use multiple BC's at different ranges to duplicate your "actual" fired drop chart, as does as Exbal. It also has G1, G5, G6, G7, Gi, Gs and Ra-4 drag curve options.

You may be right on the 1" artillery round being the foundation for the G1 drag model. I've got a half dozen articles on BC's that I've saved over the years and each has there own basis for a starting point.

I use the RSI Ballistic program. I've found it superior to any other program on the market as long as I do my part and input the data correctly.

I shoot at TacPro here in TX, which is 1000-yard range. I keep my actual data and compare it to the program. It's so close it's scary. I'm extremely confident when I go on the road for a hunt or competition that when I make my environmental changes, I'm pretty close to dead on at distances out to 750 and 800 yards.

I'm starting the range work up on my .308, which I just had rebarreled with a Mike Rock barrel. Initial results from the first 20 rounds of Hornady 168 AMAX ammo was 2 5-shot groups at 100 yards in the mid .3's and low .4's. Not to bad starting out.

Have a great weekend.

and im not sure what drag model it uses..

what do you (anyones opinion)think is the best ballistic software to use with ones own calculated b.c of a given bullet? also how would one calculate the b.c for a diffrent drag model? assuming that all given b.c's are for a G1 drag model..

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