7mm or .308?

[HuntHarder]

Man... You guys are grilling him...

[nralifer]

Why was velocity only measured with the 300 and drop with the 308? Why not both tests for each cartridge to compare results. Also how are you measuring bullet drop? Are you shooting a group and measuring to the center or only one shot? How accurate are these test rifles with the bullets in question? Are these rifles fired from a machine rest or is there a human element to be accounted for as well?

We ran out of light conditions that would allow the Oehler to work. The drop test was measured from group center. The rifles will shoot these bullets at sub minute of angle without a machine rest. By the way STOMP, did you ever try Superformance in the 7mag?

 

Lancetkenyon, I apologize for not answering your question fully. The length of the 170 bullet is 1.435 inches. It is well stabilized by a 1:10 twist barrel. Stability factor under the conditions tested was >1.4 as fired from the 308 Win. The 220 SMK measures 1.505 by comparison.

[308Nut]

On the subject of bullet length, I have a physics question for the ballistic nuts here.

 

Before you can answer the question you have to know that there are 2 bullets that are the same caliber, weigh exactly the same, have the exact same nose profile, the same boat tail profile but one is solid copper and one is jacketed lead. For obvious reasons, the all copper bullet is longer since the body of the bullet needs to be longer to achieve the same weight as the jacketed lead version.

 

With the above info in mind, which one has the higher BC?

 

After you answer, add why you picked the one you picked.

[AverageJoe]

While i am not a ballistic nut i would say the jacketed lead bullet has a higher bc. Seems if they both have the same profile the longer one would have more surface area and cause more drag along that surface area....but i am not a physics expert so i could be exactly wrong...lol

[STOMP442]

I don't own a 7mm Mag to try it in and have yet to see any superformance powder on a shelf anywhere locally so no I have not.

[Havasuhunter]

The longer bullet will have the higher BC. More bearing surface can reduce the speed you can launch it at. Long buckets with too little bearing surface can be tougher to tune and get desired accuracy and stability but have the most long range potential. I'm a 600 yard max guy, so I'm not going to get as deep into all that as some of these guys, but it's all an interesting read.

[STOMP442]

All depends on the form factors. Since form factors are determined using boat tail length and angle, ogive length, meplat diameter, radius ratio of ogive and caliber, form factors should be equal for each bullet... until we throw in drag coefficients. Since the actual drag coefficients are very hard to determine we can only compare to the standard G7 projectile. I will assume the since the solid copper bullet has a longer bearing surface than the standard G7 projectile I will also assume that it has a higher drag coefficient and thus a higher form factor and lower BC value although probably not by much.

[nralifer]

All things being equal the longer bullet will have a slightly lower BC because of frictional losses along the shank due to the extra length. However the copper bullet could actually be designed to have a significantly lower "bearing" surface contacting the rifling and be launched at a higher velocity because of lower frictional losses in the barrel. The largest effect on BC is actually the ogive profile and ogive length as well as meplat diameter.

 

Copper actually is a better material for bullet design than lead. Lighter high BC bullets can be machined and launched at higher velocities. Machining can create profiles that are difficult if not impossible to swage into lead core jacketed bullets. That is the reason why the bullets on the market have basically only 2 types of ogive, tangent and secant. Berger has a hybrid where a portion of the ogive is secant and the part closer to the shank is tangent to reduce the criticality of the bullet seating depth in influencing group size, but they all have large bearing surfaces.

[nralifer]

 

Here is a photo of some of the 30 cal bullets we have made and are now testing. From left to right, the first is the 170 grain we have recently measured a BC on. The next is a 180 grain. The third is a 220 grain SMK for comparison. The fourth is a 208 grain. With the exception of the SMK of course, all the bullets were machined from copper bar stock on a Swiss lathe. Notice the grooved that reduce the bearing surface relative to the 220 SMK. Barnes swages their copper bullets and then cuts their groves in each bullet as a separate operation to reduce their bearing surface. The groves also reduce copper fouling in the barrel because the copper displaced by the rifling deposits in the recess of each groove. Also notice the ogive shape. It is slightly different from the tangent ogive on the SMK, and theoretically more efficient at Mach 2.5 and below than either the secant or tangent. The bullets also seem to be as tolerant of seating depth as tangent ogive bullets.

[308Nut]

Can it be expected that a 170 grain that is shaped similar to the 220 SMK to have a BC higher than the 220? I can see that the nose of the 170 is slightly more streamlined than the 220 but the sectional density of the two is extremely different to the 220s favor. Sectional density and form factor are the two largest key ingredients to BC. Just because it has a form factor slightly higher than the 220 does not equate to a BC slightly higher than the 220. Specific gravity and subsequent sectional density flat prevent it. Physics cannot be cheated period. It would take the same form factor as the 215 Berger hybrid to get a 30 cal 170 grain bullet up to .550g1. We both know that the 170 does not have the same form factor as the 215. For any 170 grain 30 cal projectile to have a .65g1 it would take a form factor quite a bit north of the 215's form factor.

 

If the 170 had the exact same shape as the 215 Berger hybrid which has a legitimate .696, the 170's BC would be roughly only 79% of the 215 or about .550

 

The reality is the 215 and that 170 have different noses and boat-tails. The 170 looks to have about the same nose and boat-tail profile as the 210ABLR which has been verified by several sources including Litz to have a BCg1 of .651. With the lower sectional density, the BC will be lower as well. How much lower? About 21% lower. It is a ballistic fact that bullets of equal dimensions but varying weights change BCs proportionally to their sectional densities. 210 30 cal SD = .316. 170 30 cal SD = .256. With a form factor similar to the 208 amax, 210 vld and 210ABLR, the 170 is stuck with between .527-530g1. The longer body length to weight ratio and the addition of the grooves further reduce it's form factor. There are 168 grain bullets on the market that have BCs slightly higher than .61g1. Unfortunately, they come packaged in a box labeled 7mm. Building a 30 cal 168 class bullet to be higher than a 7mm 168 class bullet is about as difficult as developing transportation to exceed the speed of light.

 

Velocity comes in to play as you know. That said, the 400'sec or so that you'll gain over the 210s is not going to get it up to .6 let alone .65.

 

All that aside, those are some good looking bullets!

[STOMP442]

+1 to everything 308nut says. It seems we agree on more and more these days. The banding alone on the bullets pictured is huge addition to drag.

[nralifer]

There are several features of our bullets that are not obvious by simply looking at them. First of all the meplat is half the size of the SMK, and the edge of the meplat circumference is very uniform. The ogive itself is calculated from the Von Karman function, which is significantly more efficient than tangent or secant ogives. Also the ogive on the 170 bullet is longer than that of the 220SMK. Calculating the BC on the JBM Ballistics site yields a BC for the 170 bullet of 0.58 and the 220SMK of 5.7 at Mach 2.5. The big difference is the ogive length and meplat diameter , not the sectional density or even the specific gravity. The contribution to the BC of the Von Karman shape of the ogive is not accounted for in the JBM calculator because there is not one specific ogive radius that you could assign to the 170 bullet. For purposes of comparison I used the same value for both bullets. My partner, the actual designer of the bullet, has talked to Bryan Litz who estimates that the contribution of the Van Karman ogive could be between 10-14% more. 10% of .58 is 0.058 which makes the estimated BC 0.638 for the 170 bullet. Our velocity and drop measurements estimated a BC of 0.65. Looks like our data and the theory thus far agree. Admittedly we would like more data to confirm all of this and we are doing our best to get it. It is entirely possible that with more aerodynamic bullet profiles and higher energy powders the lowly .308 Win might perform the same as the 7mmMag. It sure would be interesting to find out, but as it stands right now, using CFE 223 and Leverevolution powders my 20 in barreled 308 is firing bullets of comparable weights at only about 160fps slower. One wonders what we could be doing with a 26 inch barrel.

[308Nut]

It seems like you guys are on the right track and the product looks good. You're right that if there was a 170 grain 30 cal at .6+ the potential of the 308 would be astounding. I know I'd pay an unimaginable about of money for such a product.

 

Rule number 1 when figuring BCs. Software based calculators are not reliable. Any software based results hold zero weight.

 

Rule number 2. Specific gravity and sectional density has everything to do with the equation.

 

The difference between a razor meplat and those found on match kings isn't enough to make .58 on a 170 let alone .638 or .65.

 

I challenge you to go back to your testing grounds and perform the test again using your bullets but this time shoot another projectile such as the 168 hybrid or vld side by side so you have a known reference to compare against. That way when the vld or hybrid also shows .6+, you'll know Berger is published way low, or your set up needs an adjustment. Also, send a few to another party for an evaluation.

 

What have you got to loose doing either or both?

[nralifer]

Well, actually Oehler will be shipping in March a chronograph system that sounds like it is perfectly suited for generating exactly the data both you and I want. It is called the system 88, and consists of both an optical chrono, their model 35, to determine the muzzle velocity, and an acoustic detector system consisting of 4-8 microphones, that transmits signals wirelessly over 1500 yds. Both are integrated by a receiver that then not only calculates the average BC over the distance between chrono and the microphones, but will also plot the path of the bullet as it passes the microphone array. If the microphones are arranged in a square array it can plot a group! They say that if you place a printout of the group plot on top of the actual target, the plot and the holes in the target will coincide!! You really don't even need a target. All you need is an aim point down range a little beyond your microphone array. The data is entered into your PC. The bad news is that it costs $12,500 for the basic system. They used to make a system 43 that did more or less the same thing , but discontinued it after a few yrs. It cost substantially less, apparently. I guess I will have to get one since using the Oehler 35 down range as we are doing is proving to be very laborious and slow. Bullet has to pass within a few inches of the detectors to register a signal, so it really is a hit or miss proposition, no pun intended, of course. I'm waiting for the shot where we hit the detectors.

 

Consider this. What has a higher specific gravity and sectional density, a 500 gr lead musket ball or a 500 gr Barnes 458 brass solid? I don't know off hand and just thought of the question, but clearly the shape would play a big role in determining the BC of each.

[firstcoueswas110]

question for you hardcore guys, if its all about the BC why isnt everyone shooting the same caliber and load?