175 7mm SMK real BC

[Brad Y]

Anyone got an idea of a more accurate BC figure of sierras 7mm 175gr SMK? Sierras website is a little optimistic for my liking.

[AlanF]

Bryan Litz tested them at 0.327 G7, quite impressive really. I can run them at 2950+ without brass damage, and that is ballistically superior to a 180 VLD at 2820.

[Brad Y]

Thanks Alan.

[Quick]

I did some testing early on with my 7mm with them and they showed good accuracy at short range but I opted for the 180gr VLD instead as it showed better performace. I should revisit them again as I have a few hundred left.

[Brad Y]

There are 168's here at the shop and I use them for fire forming but no need now I have a hydraulic form die coming. Just looking around really at other bullets that are available. If your willing to part with 100 Shaun I will buy them off you to play with.

[Quick]

I'll have to see how many I got left when i get home.

[aaronraad]

Anyone got an idea of a more accurate BC figure of sierras 7mm 175gr SMK? Sierras website is a little optimistic for my liking.

If you average out the 4 G1 BC figures given by Sierra from >2100 fps to <1300 fps you get a G1 BC of 0.5555, which is actually fairly conservative. Using the Berger site with their listing of G1 & G7 BC's I would only put the equivalent G7 BC @ 0.282.

Bryan's figures are normally published as an average of 4 measurements also but at 3000,2500,2000 & 1500 fps, or more correctly Mach 2.68, 2.23, 1.79 & 1.34 (as included in Bryan's texts).

Given Berger are using higher Mach numbers to derive an average G7 BC, this might explain why Alan's number from Bryan of G7 BC of 0.327 for the Sierra is effectively higher than what Sierra are quoting themselves?

Just as an aside and a general comment, it's unlikely the end result is any different, it's just that they are using two different units of measurement. A BC quoted by itself still leaves the questions of:

• which equipment was used to derived the measurements (i.e. empirical formula or a particular type of chronograph);
• over what velocities if, not Mach number(s) have they used;
• how many samples were used in the test; and
• probably a few more if you're very dependant on their published numbers, as opposed to documenting your real world range testing.

[Norm]

I had a discussion with Bryan about converting one set of G1 BC numbers for one particular projectile into G7 values and also comparing different projectiles.
Basically what came of it was that you can not convert them accurately.
Each one needs to be established independently, or so he said.
Maybe that was so you had to buy his book to get the numbers?

[aaronraad]

Plug away according to JBM :

JBM Ballistics - Drag Function Conversion

or you can go the full array:

JBM Ballistics - Drag Function Array Conversion

[Norm]

Aaron,

Interesting links. I had a play and using some basic G1 numbers and velocities it recommended a G2 BC.
The G7 BC given was in three bands and there was quite a difference between each band. This is something that might surprise some.

This highlights the common misconception that you only need one BC band for bullets when using G7 values. It also gives Bryan's comments on converting G1 BC numbers to a single G7 number some validity.

The form factor of a bullet needs to be accurate to get accurate BC predictions or it is a waste of time. Although the 175gn Sierra and 180gn Berger VLD are both boat tail hollow point bullets. If you place them side by side, there are some obvious differences in their shape and form. This will influence the accuracy of any calculated BC conversion prediction.

G7 BC numbers are not always accurate or even applicable to some match bullets. Its only that we mostly shoot at relatively short range that they seem accurate for elevation adjustments. We then tend to think that these numbers may also be accurate for comparing the wind drift of one bullet directly against another. This is not always correct, especially when the shape and weight of a bullet is different.

From my testing G7 BC numbers seem to work well for BTHP bullets that hit transonic at or near 1000 yards. For anything that can reach a lot further, I have never been able to get perfect correlation with a single G7 number using lead core BTHP bullets when shot beyond 1200 yards. The only one that did work out well was a 350gn solid copper .375 cal Cutting Edge bullet that tracked right out to 2000 yards.

From a practical point of view I would go with the one that your rifle likes best and don't stress too much about the BC numbers.

[dave]

re the 175 sierra ( or any other long range projectile) has anyone bothered to chronograph and zero them at 1000yds and then measure their trajectory height at 100yds or any other practical distance?

then compare these findings to the G1 or G7 theoretical ballistic table.

cheers
dave

[Cameron Mc]

re the 175 sierra ( or any other long range projectile) has anyone bothered to chronograph and zero them at 1000yds and then measure their trajectory height at 100yds or any other practical distance?

then compare these findings to the G1 or G7 theoretical ballistic table.

cheers
dave

Dave, I am not sure if this answers your question.
I use the JBM calculator for range wind ups. Example 300y to 1000y in 100y increments.
I find the calculations to be quite close to the real world figure.

Cheers
Cam

[AlanF]

The way that I approach BCs is to start testing a new barrel with the highest BC bullet available, then work down until good long range accuracy is found. So with 7mm I'll start with the 180 hybrid, and keep going down to 168s if necessary. I find the G7 BCs to be good enough to rank them in order, and good enough for trajectory calcs for target shooting, because we have sighters. There are so many variables apart from BC which affect trajectory that an accurately known BC is not going to guarantee anything.

[aaronraad]

From my testing G7 BC numbers seem to work well for BTHP bullets that hit transonic at or near 1000 yards. For anything that can reach a lot further, I have never been able to get perfect correlation with a single G7 number using lead core BTHP bullets when shot beyond 1200 yards. The only one that did work out well was a 350gn solid copper .375 cal Cutting Edge bullet that tracked right out to 2000 yards.

Beyond 1200y it depends on what your Mach number is like, as the average projectile is starting to go transonic. This can be compounded by the fact that the G7 BC drag curve basically isn't a good match for your projectile shape and the real velocity is not the same as the velocity your ballistics program is estimating.

Consider also how far off the curve your projectile is. A projectile with a form factor (i) of 1.000 regardless of BC means the projectile is essentially an exact match to standard projectile for that drag curve. There is nothing to state that as a projectile deviates from a form factor of 1, that it does so in any proportional manner. Further to this is the assumption that this deviation applies across the entire velocity and trajectory curve.

The other element might be something else we take for granted at times, the projectile's average density or more correctly stated in specific gravity. Note the data input variable in JBM Calculator - Drag/Twist. Specific gravity is relatively simple to account for in a monolithic solid, not so easy for a cup & core hollow point with 3 different media of differing specific gravity. You must also account for the variable thickness of jacket walls between bullet batches. Maybe you can pop a bullet into a glass of water and see how much water it displaces, in a temperature controlled environment and then weigh that sucker, but how do you account for the open tip air space? But what is the specific gravity of the air in the open tip at 3000 fps, as surely it must be compressed somewhat. The air gap in the open tip can also be significant if you've seen the volume in a high BC cup & core 155gr 308 Palma projectile. I'm possibly over-thinking it but it wouldn't the first time.

[pjifl]

I will just throw this in because some of this discussion is verging on critically measuring a projectile.

I have found that the bast way to reverse engineer a projectile is to draw it up and calculate all the parameters like volume, Moment of Inertia, Mass (which is a cross check) and Centre of Gravity.

1/ Set a digital caliper to different diameters and gently rotate the projectile in the jaws. This will leave rings marked on the copper. Measure their longitudinal position then draw and model it and calculate.

2/ Reach into the hollow tip with a piece of wire to feel tor the lead termination. Its position will often surprise you.

Trying to measure displaced volume is not very accurate because of surface tension issues.

There are other ways to reverse engineer an Ogive but the above always works and for a one off measurement is probably as fast.

Finally, I agree with some comments that exact BC and quibbling about small discrepancies is a bit pointless.

Peter Smith.