Bullet base-to-ogive differences and a question.

[AlNyhus]

This gets cussed and discussed a lot. I wanted to post this example because it might clear some things up for those that measure this stuff.
-The bullet on the left is .340 B-O, he bullet on the right is .390 B-O. That's .050 difference in B-O.
- Both bullets were made on the same dies.
- Using the accurate Touch Point (aka 'Wheeler' method) in the same barrel, how much seating depth change will be needed between bullets to get to the T.P. (Touch Point)?

Good shootin' -Al

 

[6ShotsOr5?]

I’ll give it a shot Al.

Both will touch the lands at the same CBTO measurement. The base of the longer bullet will have to be seated .05 in deeper into the case for the cartridges to have the same CBTO.

 

[K22]

They look like different bullets, at least optically.

"Apples to oranges" comparison making the differences irrelevant?"

 

[Chilly807]

If you're asking what I think you are, I don't think you change the seating die setting at all to get the same jump (or jam).

If the bullets come from the same die, the point on the nose where the seater plug touches is the same on both bullets, and so is the distance from that point to the ogive where it touches the lands.
The bullet on the right will be seated deeper in the case.

I think I'm saying the same thing 6shotsOr5 is.

 

[Lizzy49]

As long as you use the same bullet seating die no depth change would be needed.

 

[Bill Norris]

As long as you use the same bullet seating die no depth change would be needed.

True, unless the load is heavily compressed with lower neck tension wherein the powder charge will have some degree of push back pressure on the base of the bullet. Have seen this especially in .308 compressed loads.

 

[TTE]

As long as you use the same bullet seating die no depth change would be needed.

I vote for this answer

 

[RegionRat]

The answer isn't that the two will use the same 0.050" difference with a barrel unless the barrel and the tool used to measure that base to ogive value happens to have the same effect as that tool.

Comparator tools have an edge contact, and rifle bores have what we would call an interrupted cone surface. Unless you create a comparator tool that also matches that rifle's bore, then the delta is not going to be 0.050".

If the two bullets share the same ogive die, then they should result in a near identical COAL at touch, even though it won't be related to the value of the comparator tool.

 

[Richard Jones]

Unless you create a comparator tool that also matches that rifle's bore,

Done that!

 

[jackieschmidt]

Seating depth is the measurement from the bolt face to the first point of bullet contact in the barrel.

All other things a being equal, the seating depth will be identicle.

 

[DShortt]

Seating depth is the measurement from the bolt face to the first point of bullet contact in the barrel.

All other things a being equal, the seating depth will be identicle.

I'd agree. The bullet will be seated deeper into the case in the bullet with a longer ogive to base measurement. Which changes case capacity. But that's a separate discussion.

 

[JFrank]

Agreed, if the nose geometry is equal on both bullets they should seat to the same CBTO.

 

[DShortt]

This gets cussed and discussed a lot. I wanted to post this example because it might clear some things up for those that measure this stuff.
-The bullet on the left is .340 B-O, he bullet on the right is .390 B-O. That's .050 difference in B-O.
- Both bullets were made on the same dies.
- Using the accurate Touch Point (aka 'Wheeler' method) in the same barrel, how much seating depth change will be needed between bullets to get to the T.P. (Touch Point)?

Good shootin' -Al

What is it you're not telling us?

 

[AlNyhus]

What is it you're not telling us?

Just thought it was a good visual example.....

 

[Jeff Porter]

So...Please correct me if I am failing to understand the concept.

If I am reading and seeing this correctly, the profile ( the arc of the bullet from the bullet tip to the ogive) is identical in these two bullets as they came from the same forming die. One is longer and has more bullet bearing surface from the base to the ogive. The 0.050" difference noted is only in the back end of the bullet. So using the Wheeler method, where the touch is designated as the cartridge case base to bullet ogive (CBTO) distance at the point the ogive first reaches the rifling, since the tip to ogive profiles are identical, the CBTO would be the same as well with the longer bullet extending 0.050" deeper into the case neck.

The confusion comes when trying to say a micrometer seating die set at X.XXX" for a CBTO for one bullet type will give the same CBTO for any bullet. In this case, where the tip profiles are the same it should. BUT... The different bullet profiles of different bullets from ogive to tip will contact the bullet seater stem at different points along the profile. Long pointed VLD secant ogive bullets will bottom out and the tip will contact the base of the seating stem in most standard stems.
Short stubby bullets will jam deep into the rifling if you use a long tipped bullet seater die setting that is well off the touch point for the longer pointy bullet. ( try to load .223/5.56 Wylde chamber with a 52 gr bullet using a 77 gr mag length die setting and open the action, stuck bullet and powder every where... ask me how I know)

Combine this with the variances in the inside diameter of the comparator tools used to measure CBTO causing these to contact the bullet at locations the than the actual ogive diameter ( closer to the tip) and it makes any CBTO measurement taken or seating die setting a very specific number for your rifle, your tools, and your lot of bullets.

This is why even using the Wheeler method, you will get slightly different measurements of what should be a fixed distance from the face of the bolt to the touch point of the barrel using a different bullet. The more precise you tools the more consistent that number but factory Hornady type comparators will vary from bullet to bullet and even sometimes lot to lot of the same bullet due to slight differences in that bullets profile.

Edit to add, I didn't say anything that hasn't already been said in this thread, just wanted to make sure my pea brain is on the right page.