Settle this for me! Friendly Debate

[Bullet bumper]

Where a certain bullets surface contacts the lands is dependent on shape of the ogive curve and at what point on that curve the bore diameter comes up . Long slender ogives will go further into the bore before the bore diameter jams . Short blunt ogives will go less into the bore . It will always jam at the bore diameter but the length from there to the base will vary between different style bullets .
That is why you may get one particular style of bullet to the lands and not some other styles. Flat base gives more seat out room than a boat tail of the same ogive shape and same total length .

 

[Crowe284]

Yessir, I get this. I'm generally talking about moving from a 168 VLD to a 180 VLD etc. Not VLD to flat base partition or something like that.

 

[Bullet bumper]

I'm not suggesting you use anything , just trying to explain in a simple way . If you have both bullets and mark on the ogive where the bore diameter starts then lay them side by side with a loaded round having the old bullet seated and you can see how much shank will be available for seating depth of each bullet .

 

[Edd]

Yessir, I get this. I'm generally talking about moving from a 168 VLD to a 180 VLD etc. Not VLD to flat base partition or something like that.

A VLD bullet does not make initial land contact at bore diameter. They contact the lands at the bullet bearing surface diameter.

If the bullet diameter is the same and you switch bullet weights, the distance from the cartridge base to the point where the bullet contacts the lands will be that same.

The distance from the cartridge base to the point you are measuring may or may not be the same depending on the bullet nose shape and length.

The two bullets you referenced happen to have noses that are, according to the Berger spec sheet, almost identical.

 

[Bullet bumper]

A VLD bullet does not make initial land contact at bore diameter. They contact the lands at the bullet bearing surface diameter.

That's not correct . The lands are the bore diameter not the groove diameter .
The bullet will stop some place on it's ogive shape at that bore diameter of the front edge of the lands if the lands are not eroded away that is.
If a barrel is say .308 groove diameter size of the bullet , then the lands are around .300 diameter roughly speaking so the bullet jams some place because it's shank is .308 . There is only a few thou in it but it is on the edge of the ogive .

 

[Edd]

That's not correct . The lands are the bore diameter not the groove diameter .
The bullet will stop some place on it's ogive shape at that bore diameter of the front edge of the lands if the lands are not eroded away that is.
If a barrel is say .308 groove diameter size of the bullet , then the lands are around .300 diameter roughly speaking so the bullet jams some place because it's shank is .308 . There is only a few thou in it but it is on the edge of the ogive .

When the throat is cut, the lands extend from the groove diameter to the bore diameter. If the chamber throat is cut at 1-1/2° with an ID of .300" and an OD of .3085" and your bullet has an OD of .308" and the nose has a taper of anything more than 1-1/2°, the contact point will be closer to the groove diameter than the bore diameter. A Berger VLD Bullet will have a nose taper of ~10°.

 

[Crowe284]

Man, you guys are smart! Trying to follow alll this without taking out a scientific calculator! Love it!

 

[Bullet bumper]

You just said it , " Closer to it but not actually groove diameter "

 

[Edd]

You just said it , " Closer to it but not actually groove diameter "

What will be the diameter of the contact point?

 

[barefooter56]

Clear as mud! Ha! So....Lets say you are using nothing but berger VLD type bullets and just switching weights, you ought to be **** close?

As for the comparator issue....I have a GAP built 6.5 4S that I bought from a forum member. Also bought all of his loaded rounds that shoot bug holes. His CBTO measurement differed from mine as I wanted to document his exact jump (which was .001) when I started loading. Although we got two different CBTO measurements, as long as I used MY equipment on loaded rounds and duplicated using MY measurements I should end up with same jump, correct? Have worried about this ever since I found the load he developed was unbelievable and wanted to duplicate. I measured everything on the loaded rounds (headspace etc. etc.) and on fired brass so that I would have all of this to duplicate. Even though he did the loading, if I use my measuring tools and use those measurements, I think I should be close....hopefully?

correct. Difference would be in the bullet comparators. Yours vs his. Measure his rounds and use your numbers.

 

[barefooter56]

So a friend and I were having a conversation today about distance from base to ogive measurements. He proposed that the ogive is what contacts the lands first and that when you are using a 175 berger elite, or a 180 vld or even a 195 EOL that when you find the lands with any of these bullets that the measurement that you get (base to ogive) will be the same with any of the other bullets and that the only thing that will change that none of us probably never use is OAL from base to tip of bullet for the longer bullets as you go up in weight. What say you to this statement? Now, this is assuming that you are using your calipers with your comparator installed for that particular caliber, i.e., all measurements are using same measuring components.

SETTLE THIS FOR ME!

Crowe 284,
You have three different bullets so probably not. What MAY end up being the same is the accuracy seating depth on or off the lands though. Example: if the VLD bullet likes it .010 off the lands the other two bullets may as well also. You have to make a "dummy round to the lands to get their CBTO to the lands though to find what their CBTO .010 off the lands would be. When you change to another bullet you should always make a dummy round to the lands for that bullet. I write the bullet manufacturer, type caliber, weight and what the CBTO is on the cartridge case of the dummy round with a sharpie to keep them straight.

 

[Crowe284]

Thanks for the response Phil, that made sense to me!

 

[barefooter56]

Glad to help!