189 Bob Caurterucio BCS?

[toddconley]

Could someone help me out with the Bcs on on the new 189 bullets from Bob. I am having a hard time calculating drops. On a side note. I am running these from 280 ai at somewhere around 2840. Is this fast enough to drive a bullet this big for maximum ballistic advantage or do they need to be driven faster to take advantage of the size? They absolutly shoot like a house afire at this. Which is 55 gr of rl 22 under them. But, it seems they are not as ballistcally superior as one would think. It takes about 11.75 moa to get to 575 which as far as I have shot them so far. But they will hold 1.5-2.0 group at that range. They also cut the wind awsome. So I guess I will stick with them. I know there are several people here who know the science of this sport and understand things like this so chime in if you have a EDUCATED opinion.
Thanks,
T

 

[Rtheurer]

Hey Todd

How do I find some of Bobs bullets. I did a web search and found nothing.

Sorry to highjack your thead

RussT

 

[toddconley]

bcbullets@sbcglobal.net
650-583-1550
482 Comerwood Ct.
South San Francisco CA 94080-4205

 

[TonyR]

Todd. For what it is worth, the come-up from a 100 yard zero to 575 with a bullet with a .600 BC with a 2840 MV is 11.50 MOA. The Berger 180 VLD has what I have found to be real .659 BC. I have always heard that Bob's bullets have a higher bC than the Berger so you might want to get a solid MV. My Shooting Chrony always read high which would make the BC seem lower.

 

[toddconley]

If you could take two drops at two know ranges could you calculate mv and bc or would you need one of the numbers?
Thanks
T

IE drop is 6.75 @ 425 and 11.75 @ 575

 

[TonyR]

Todd.
I just e-mailed a friend that shoots them in his straight 280 out to 1000 yards. He knows his velocity and sight adjustments for 600 and 1000. I'll try and figure it from that when I hear back from him. I'm kind of curious about the BC myself. I have 100 of them but never shot any.

 

[TonyR]

Per EXBAL, .760 BC, 2840 MV, 100 yard zero, 10.75 MOA up for 575 yards.

 

[skinnj1]

What twist is required to stabilize the 189's?

 

[toddconley]

Thanks to all. I went out today to test further distances but couldnt get set up as far as I wanted to go. Have to wait until the ccorn is shelled to get any farther than 575.

 

[Heman]

I just recieved 50 of Bob's 189's and 50 of his 178's to test. I used the form factor equation in Bryan Litz's book to calculate the G7 BC for the bullets. The values I calculated were .312 for the 178's and .327 for the 189's. Not exactly what I was expecting however this is due to the large Meplat on the bullets. Once I pointed the Meplats the form factors dropped and thus the G7 coefficients increased dramatically giving BC's of .352 and .364 resectively. For compairson the 175SMK is .327 hopefully will get a chance to test soon but based on the numbers they look impressive.

 

[Bryan Litz Ballistics]

Put in 2840 for the velocity .760 for the bc zero the gun at 100 and see what you get at 600 yards.Remember you were shooting 575 but it will be close enough for government work.

The drop for a bullet/MV like this will be around 68.75' at 575 yards, and 76.71' at 600 yards. If you pretend that 575=600 yards, it will cause you to calculate a BC that's in error by over 50%.

When measuring BC's, it's very important to measure all the variables with as little uncertainty as possible because small errors can make a big difference.

I'm not saying these bullets don't have a high BC. They probably are higher than other heavy weight 7mm's since they're heavier. Shawn's calculated BC suggests a G7 form factor of .95, which is very typical of this bullet shape. His G7 BC of .352 would translate into a G1 BC of roughly .688,average from 3000 fps to 1500 fps)

Take care,
-Bryan

 

[toddconley]

I feel kinda special that I have this kind of talent chiming in on this question I have posed. I still have not had time to test this futher but hopefully soon I will check the fps at 100 and the fps @ 200 and we will see exactly what is going on. I will up date at when I get this done. Anyway you go these are some crazy acurrate bullets in my stick.
T