B.C. on two 7mm Remington Core-Lokt bullets. I'm confused

[Newtosavage]

My Ruger 77 Mk II in 7x57 loves the factory loaded "green box" Remington ammo, with the 140-grain Core-Lokt bullet. That's the good news. The bad news is that those boxes of 7x57 are expensive and hard to find, and I can't find those bullets available anywhere. So I noticed that Midway had some bulk 150 Core-Lokt bullets in 7mm and I thought I'd give them a try.

What I can't understand is that according to Remington's own ballistics chart, the 140 grain Core-Lokt has a HIGHER ballistic coefficient (.39) than the 150 grain (.346). That just doesn't make sense to me. Same diameter, longer bullet should have a higher BC, right?

https://support.remington.com/CORE-LOKT®_CENTERFIRE/Rifle_Ballistics_for_Core-Lokt:_7mm_Mauser_(7x57)_-_30-40Krag

Any thoughts?

 

[mavracer]

BC depends a lot on shape of the bullet the 150 may be less pointed with a flat base and the 140 may be more pointed and have a boattail. Have no idea as to actual reason though can't find pictures of both bullets

 

[stoky]

Possibly a velocity constraint (?)
I've had great luck with Core-Lokts on game. I wouldn't sweat the published BC too much.
Not sayin they'd do marketese specsmanship with the numbers or nutin. , but with the affordability of doppler chronys getting better, things are getting interesting.

 

[Newtosavage]

Both bullets are flat based.

Core-lokt have a great reputation on game which is why I bought the bulk 150's. Was thinking of using them for elk out of my .280 at around 2900 fps. if they grouped well enough. But with that BC, I could pick a 150 Partition and have a lot more energy at 400 yards.

 

[stoky]

For elk, I'd probably go with the Partitions, they're very good too. See if you can consistantly get them on a 10" plate at 400yds.

 

[Newtosavage]

I most likely will do just that.

I tried these 150 Core-lokt's out of my 7x57 and they weren't magic like the 140's are. So I figured I'd load them in my .280 and see what I could get out of them. I like the bullet, but you're right - partitions would probably be the better choice.

 

[Sunray]

Really isn't something to worry about. BC is a measure of how streamlined a bullet is and you'll likely find a formula(that'll give you a nose bleed) in your manual's Reference chapters.
Anyway, a 140 will be a tick shorter than a 150 and the BC for either of 'em will change according to the temperature, altitude and barometric pressure, etc.
400 yards is too far for either a 7mm Mauser or a .280. 140's and 150's drop like bricks past 300.

 

[Newtosavage]

400 is too far for a .280? You do realize it's a necked-down '06 right? Some of my handloads are sending 160 grain Accubonds at over 2800 fps. from just a 22" barrel.

Anyway, I didn't start this to argue effective range but rather to see if anyone had some insight on why the longer bullet might have a lower BC.

 

[Laphroaig]

BC is the ratio of Sectional density divided by coefficient of form. The heavier bullet will have a greater sectional density, and if the 2 bullets have approximately the same shape I'd assume their coefficient of form will be similar. Therefore the bullet with the higher SD will also have a higher BC.

That's my story and I'm sticking to it!

 

[Newtosavage]

BC is the ratio of Sectional density divided by coefficient of form. The heavier bullet will have a greater sectional density, and if the 2 bullets have approximately the same shape I'd assume their coefficient of form will be similar. Therefore the bullet with the higher SD will also have a higher BC.

That's my story and I'm sticking to it!

Well, that's what I would have expected as well but none of the published charts for the 150 7mm Core-lokt support that idea, for whatever reason. Maybe it's a typo?

 

[Newtosavage]

For comparison's sake, the 150 7mm partition shows a BC of .456. I'm beginning to wonder if the real BC for the core-lokt isn't .446 and that it was a typo. Next trip to the range I should be able to get some data at 100, 200 and 300 and map it out.

 

[Newtosavage]

I think I have my answer. Pretty sure that's a typo. Sighted in at 200 today @ 2800 fps., the group was 7" low at 300 suggesting a BC of about .450

Right at 1" group at 100 yards. Should make for a great all-around load.

 

[Gtscotty]

400 is too far for a .280? You do realize it's a necked-down '06 right? Some of my handloads are sending 160 grain Accubonds at over 2800 fps. from just a 22" barrel.

Anyway, I didn't start this to argue effective range but rather to see if anyone had some insight on why the longer bullet might have a lower BC.

I would be very happy with 160gr Accubonds @ 2,800+ fps for elk, why even bother with any 150's at that point?

 

[Newtosavage]

cuz they are cheap and I like to shoot?

 

[old heeler]

400 is too far for a .280? You do realize it's a necked-down '06 right? Some of my handloads are sending 160 grain Accubonds at over 2800 fps. from just a 22" barrel.

Anyway, I didn't start this to argue effective range but rather to see if anyone had some insight on why the longer bullet might have a lower BC.

I'm big fan of the 7mm. I have some Rem 140gr/150gr and the 150gr length is 1.177" and 140gr is 1.115" and that a difference of .062". I like the 150gr as it has longer bearing surface, seat bullet out and I shot those in 280AI.

I'm not really sure short yardage the .044 BC difference in those bullets is really something to worry about.

 

[LoonWulf]

Just a note litz came up with .425 bc for the 150 partition. Honestly 400yds and in im not horribly conserned with bc as long as its over .350.
My grendel with a .350ish bc and 2750-2800 hits stuff at 300-400 without alot of effort.

As a note my favorite bullet weight for the larger 7mms are 160-168s. I LOVE the 162Amax, and 168 Bergers for deer size game.
While ive never shot elk, ive shot a couple cows with the 162s and 175SGKs from my STW and Rem mag, penetration was plenty good enough on both and soft tissue damage was significant enough to lay them out inside of 20yds.

 

[Newtosavage]

I have no idea why a 150 core-lokt would have a significantly different BC than a Partition. If anything the core-lokt looks a little more aerodynamic but I don't know that for sure. If I can get my chrono working again, I'll run a few over it at 300 yd. and see what the velocity is, then it should be easy to run the #'s and see.

I know this sounds trivial, but I have a target # for ft. lbs. that I would be confident in using and that helps dictate my shot limitations. Not everyone will think that's necessary, but it gives me a good ballpark range to start with.