7mm 160 TMK experiences

[solarshooter]

That's not quite right. Trajectory depends on the bullet's BC, which depends, in part, on the bullet's form factor, but also on its SD. The proportionality constant for both form factor and SD is 1 (an equal dependence on both).

That's a really good point. What I'm after is sort of "BC per unit weight" which I take as a proxy for "BC per unit recoil". So if I write it out:

BC = SD/i

i = form factor
SD = bullet weight/bullet diameter^2 = w/d^2

BC = w/(d^2*i)

So "BC per gr" = BC/w = w/(w*d^2*i) = 1/(d^2*i)

Does this seem like the right factor? Bullet weight cancels out here, which is a little counterintuitive. Overall ranking for a given caliber doesn't change.

 

[Jordan Smith]

That's a really good point. What I'm after is sort of "BC per unit weight" which I take as a proxy for "BC per unit recoil". So if I write it out:

BC = SD/i

i = form factor
SD = bullet weight/bullet diameter^2 = w/d^2

BC = w/(d^2*i)

So "BC per gr" = BC/w = w/(w*d^2*i) = 1/(d^2*i)

Does this seem like the right factor? Bullet weight cancels out here, which is a little counterintuitive. Overall ranking for a given caliber doesn't change.

Ah, I see what you're trying to do. Makes sense. The only thing I'll add is that there is a missing pre-factor of "4/pi" in the numerator on the right side of the equation. So it becomes

BC/m = 4/(i*pi*d^2)

 

[solarshooter]

Ah, I see what you're trying to do. Makes sense. The only thing I'll add is that there is a missing pre-factor of "4" in the numerator on the right side of the equation. So it becomes

BC/m = 4/(i*d^2)

Is that because SD should be 4*w/(pi*d^2)? If so the factor would be 4/pi right? But in the definition of SD for ballistics, they drop that scalar 4/pi and just use w/d^2. So I don't think any factors are needed, unless you're referring to something else.

 

[Jordan Smith]

Is that because SD should be 4*w/(pi*d^2)? If so the factor would be 4/pi right? But in the definition of SD for ballistics, they drop that scalar 4/pi and just use w/d^2. So I don't think any factors are needed, unless you're referring to something else.

Yup, exactly. I missed the pi.

That's an oversight on the Wikipedia BC page. If you go to the SD page, it's defined properly.

Sectional density - Wikipedia

en.wikipedia.org

EDIT: it seems to be defined without the 4/pi pre-factor in another resource I found, as well. I'm not sure why, other than to simplify, given that 4/pi is slightly over 1, so they may just re-normalize to make things simple.

 

[solarshooter]

Yup, exactly. I missed the pi.

That's an oversight on the Wikipedia BC page. If you go to the SD page, it's defined properly.

Sectional density - Wikipedia

en.wikipedia.org

In absolute terms I agree with you the definition with actual area calculation including 4/pi is correct. But I'm pretty sure that within ballistics in particular, what I said is true, since most published BC, i7, and SD values agree with the definition I wrote. I'm also pretty sure Litz defines it the way I wrote in his books, as does Bob McCoy. Not sure why they do this, other than for simplicity, since the whole BC with form factor method is itself an approximation of a real Cd vs Mach curve.

Litz:


McCoy:


So it's a very specific, niche, and incorrect definition haha.

 

[Jordan Smith]

In absolute terms I agree with you the definition with actual area calculation including 4/pi is correct. But I'm pretty sure that within ballistics in particular, what I said is true, since most published BC, i7, and SD values agree with the definition I wrote. I'm also pretty sure Litz defines it the way I wrote in his books, as does Bob McCoy. Not sure why they do this, other than for simplicity, since the whole BC with form factor method is itself an approximation of a real Cd vs Mach curve.

Litz:
View attachment 877790

McCoy:
View attachment 877791

So it's a very specific, niche, and incorrect definition haha.

Yup, I think you're right about that. I agree that it's probably an approximation for simplicity.

And it appears to only be the case in external ballistics. Seems as though it's defined properly in terminal ballistics and medical ballistics. Interesting discussion on the topic here: https://www.shootersforum.com/threads/sectional-density-formula.77211/

 

[Mtaylor]

Midway USA is having a sale in 160 gr TMK’s right now. A box of 100 for $29

 

[Qholum37]

Does anybody have any of these on hand to pull some quick measurements?

 

[KY_coop]

Depending on the measurements you need i can take a look at them

 

[Qholum37]

Depending on the measurements you need i can take a look at them

That would be awesome! Overall length, boattail length and base to ogive.

 

[Kimber7man]

Midway USA is having a sale in 160 gr TMK’s right now. A box of 100 for $29

Thanks

 

[KY_coop]

 

[Qholum37]

View attachment 892307View attachment 892308View attachment 892309

Excellent info! I haven't been able to find these dimensions anywhere online so I'm sure this will help many others in the future.

 

[SonoranPrecision]

I picked up a couple 500 packs of the 160 when they were on sale a month ago, and just opened them up to find the tips look like shit right where they meet the jacket. Not sure if this will affect precision or bc consistency in any way, but I guess I’ll find out. Pretty much every single tip is flawed to some degree. It’s honestly hard to find one that looks “right”. These weren’t advertised or sold as seconds, but they sure look like it.

 

[Qholum37]

I picked up a couple 500 packs of the 160 when they were on sale a month ago, and just opened them up to find the tips look like shit right where they meet the jacket. Not sure if this will affect precision or bc consistency in any way, but I guess I’ll find out. Pretty much every single tip is flawed to some degree. It’s honestly hard to find one that looks “right”. These weren’t advertised or sold as seconds, but they sure look like it.

View attachment 908626

I was going to pick some up on that same sale and after seeing this I’m glad I didn’t! I’ve never seen tips that bad on TMK’s before. Maybe reach out to Midway and see if they’ll exchange them?

 

[rickyw]

I picked up a couple 500 packs of the 160 when they were on sale a month ago, and just opened them up to find the tips look like shit right where they meet the jacket. Not sure if this will affect precision or bc consistency in any way, but I guess I’ll find out. Pretty much every single tip is flawed to some degree. It’s honestly hard to find one that looks “right”. These weren’t advertised or sold as seconds, but they sure look like it.

View attachment 908626

I just bought a box of 500 from midway as well but haven’t opened them up yet. I’ll have to take a look when I get back. Yikes

 

[SonoranPrecision]

I was going to pick some up on that same sale and after seeing this I’m glad I didn’t! I’ve never seen tips that bad on TMK’s before. Maybe reach out to Midway and see if they’ll exchange them?

Yeah I’ve reached out to both Midway and Sierra, so we’ll see what they say

 

[rickyw]

My box of 500 look terrible as well

 

[SonoranPrecision]

My box of 500 look terrible as well

Good to know

 

[SonoranPrecision]

As an update, Sierra is sending a complete replacement of what was purchased. They also said to keep the bad boxes and sort them, use them for foulers, whatever. Pretty excellent customer support