Question on CBTO measurement

[BirdDog50]

Hello, need your expertise on CBTO measurement. Currently loading 300wm with Berger 210gr VLD' s at 0.020 thou off lands which measures 2.7765. I'm looking to test some Berger 220gr hybrids with same powder IMR 7828SSC 68.5gr...
The 220's measure 2.816 to the lands so 0.020 off equals 2.769. The test loads fit the magazine & cycle through the action without any resistance . So is the longer CBTO measurement because the 220gr bullets are longer and therefore thinner?
I think I'm safe to test these loads but wanted confirmation if possible. Thanks

 

[338 dude]

.

 

[Justice1327]

I don't know about thinner… But it simply means that the 220 grain has a different point of contact where the ogive begins. Given the measurements you stated it seems like the 220 grain has a shorter CBTO…Not longer.

 

[338 dude]

I don't know about thinner… But it simply means that the 220 grain has a different point of contact where the ogive begins. Given the measurements you stated it seems like the 220 grain has a shorter CBTO…Not longer. Plus I think your math is off it should be 2.616 if it is 20th .

 

[BirdDog50]

338...sorry for any confusion...the 220gr measures 2.816 to the lands so thinking minu 0.020 equals 2.796.

 

[338 dude]

338...sorry for any confusion...the 220gr measures 2.816 to the lands so thinking minu 0.020 equals 2.796.

Got an extra digit on one measurement and believe me I am no math scholar I'm easily confused lol

 

[338 dude]

Your post stated 2769 not 2796 I thought I was screwing up on the math so I just deleted what I said lol no biggie I'm pretty dyslexic myself

 

[VLD Pilot]

Hello, need your expertise on CBTO measurement. Currently loading 300wm with Berger 210gr VLD' s at 0.020 thou off lands which measures 2.7765. I'm looking to test some Berger 220gr hybrids with same powder IMR 7828SSC 68.5gr...
The 220's measure 2.816 to the lands so 0.020 off equals 2.769. The test loads fit the magazine & cycle through the action without any resistance . So is the longer CBTO measurement because the 220gr bullets are longer and therefore thinner?
I think I'm safe to test these loads but wanted confirmation if possible. Thanks

2.796 is .020 off

 

[VLD Pilot]

Hello, need your expertise on CBTO measurement. Currently loading 300wm with Berger 210gr VLD' s at 0.020 thou off lands which measures 2.7765. I'm looking to test some Berger 220gr hybrids with same powder IMR 7828SSC 68.5gr...
The 220's measure 2.816 to the lands so 0.020 off equals 2.769. The test loads fit the magazine & cycle through the action without any resistance . So is the longer CBTO measurement because the 220gr bullets are longer and therefore thinner?
I think I'm safe to test these loads but wanted confirmation if possible. Thanks

Possibly the nose is longer adding the extra 10 grains up front. I'm guessing the base to ogive is a bit longer as well.

 

[LaHunter]

One thing to keep in mind, the distance to the lands is going to continue getting 'longer' as you put rounds down the barrel due to erosion of the lands.
Also, different bullet types/weights will have different CBTO measurements. Even different lot #s of the same bullet type can have different CBTO measurements.

 

[VLD Pilot]

All you can do is measure what you have even if they are different lot #s. Weight should be the same and if the measurement is the same, you should see good results. As a hunter, I'm not as concerned with this so much. Comp shooters load closer to the lands even in them. That's where measuring can really make a difference.

 

[Justice1327]

2.796 is .020 off

Correct…I hate public math!

 

[BirdDog50]

Thank you all for the great intel & confirmation I apologize for my dyslexic numbers ( 2.769 should have been 2.796) in my original post. The mind is a terrible thing to waste or I'm just getting old

 

[VLD Pilot]

Thank you all for the great intel & confirmation I apologize for my dyslexic numbers ( 2.769 should have been 2.796) in my original post. The mind is a terrible thing to waste or I'm just getting old

You never know, that 2.769 may be where your rifle likes the bullet. I have a few that jump over .100 with very good accuracy.