Bob Green Comparator and bullet sorting

[dmoran]

Kyle

The BGC measures my E to F references.
Example: for a 6mm/243 BGC it measuring from a diameter of say.236" to again at say .215"
(Don't have one, so I'm just guessing at the above BGC diameters, but of similarity regardless)

Good Luck
Donovan

 

[dmoran]

gstaylorg -

Good read..... !.!.!

Couple quick notes from me:
- any and all variation in ogive lengths/diameters will also result as a variation in BC
- a shorter bearing bullet takes less force to engage the rifled bore then a longer bearing surface bullet. But once either are engaged in the rifling, they push down the extent of the barrel with fairly equal force (or so is my own determinations from driving bullets into barrels and pushing them through).

Donovan

 

[jlow]

Good discussions indeed! A couple of questions:


In terms of bearing surface length, what is its potential effect on interaction with the case neck? Its seems changes in length of case neck affect precision but changes in the length of the other interacting surface i.e. bearing surface could do similar things?


What is the history of variation of a high quality bullet like a Berger in the different areas of the bullet i.e. shape of its ogive vs. bearing surface length assuming that all bullets in a batch comes out of the same die? Has anyone done a survey? It seems like although any change is possible, in actuality some areas are probably more susceptible due to the way the bullets are manufactured?

 

[skiutah02]

As illustrated very nicely in Donovan's bullet images above, there are a number of length target areas people measure for the purpose of sorting their bullets, most commonly OAL, base-to-ogive, ogive, or bearing surface lengths. None of these can be considered "perfect", because measurement of any one does not rule out the possibility that there could be variance in one or more of the others. The whole point of sorting anything is to improve consistency, which theoretically will pay dividends in terms of better precision.

The question asked in the original post referred to how sorting by bearing surface compared to sorting by ogive using the Bob Green Comparator tool. This question has been effectively addressed above by a number of individuals that have actually directly determined whether they could shoot the difference in bullets sorted by BCG alone versus those sorted by both bearing surface and BCG. The answer was that BCG sorting alone was sufficient for optimal consistency. However, there still seems to be some confusion as to exactly what is being sorted with the BCG and why it is better to do it that way, so here is a slightly different way of thinking about the process.

When we have identified an optimal charge weight and are carrying out seating depth testing, clearly we are changing the internal case volume to some extent by how far down into the case neck the bullet shank/boattail is seated. I find I can typically move a bullet seated out of the lands by at least .010" to .015" in either direction without changing the velocity sufficiently that I can reliably measure a change. Almost every bullet I use has shown optimal seating depths in the range of ~.010" to ~.025" off. For that reason, I typically do charge weight testing at .015" off the lands. Then I can test seating depth in .003" increments from ~.006" off to ~.024" off and will only be moving the bullet by a maximum of ~.009" in either direction. As I mentioned, I routinely determine velocity during seating depth testing and as long as the bullet isn't close to (or into) the lands, the velocity just doesn't change enough my chronograph can reliably detect a difference.

The point of this is that if we can move the base of the bullet in or out of the neck by these amounts (i.e. change the effective case volume) without dramatically affecting velocity, causing the same effect by slight changes in bearing surface length are unlikely to have a demonstrable effect on velocity either. It is worth noting that changing the bearing surface length could also affect velocity via increasing/decreasing friction with the lands. However, changes of a few thousandths in bearing surface length are going to change kinetic friction very little due to the small surface area that actually contacts the lands and the fact that the coefficient of static friction is typically larger than the coefficient of kinetic friction.

So, if minor changes in bearing surface length are unlikely to cause significant changes in velocity (i.e. consistency), what is a more likely target measurement that we can effectively control? Of course, the answer is seating depth. Seating depth affects barrel timing, and even small changes in seating depth can have a significant effect on precision. The problem inherent to precise measurement of seating depth lies in the fact that most seating die stems contact the bullet well out toward the meplat relative to where the caliper inserts we use to measure CBTO seat. Any variance in between these two contact points means potential variance in seating depth and/or how the bullet enters the lands. Bob's comparator allows sorting of bullets into groups where the difference between these two contact points is minimized, thereby improving consistency.

Easy to "like" this analysis. Dead-on in my opinion.

 

[dmoran]

jlow -

Hopefully Bart or another bullet maker chimes in.... but from my own speculation and perspective, the ogive shape (diameters at given lengths) would be more susceptible to variations then the bearing, from being folded/formed over in creating the nose. Which could be why I see most bullet variations from the end of the bearing to the meplate, and very little to nil before the bearings end.

Donovan

Edit:
I have made myself comparator plates with multiple holes that I use in part to profile bullets. When segregating bullets by my above referenced methods, I have compared extremes against each other by profiling the entire bullet, and is where I base my inputs to bullet variations from.
Below is an example of a bullet profiling that I did for Bart last spring for his 105-RBBT:

 

[jlow]

That's good work! Thanks Donovan!

 

[Kyle Schultz]

Great stuff guys. Thanks for the info.

 

[Kings X]

Is seating differences of .003" that important when I am "jumping" the bullet .010"?

 

[Cigarcop]

Is seating differences of .003" that important when I am "jumping" the bullet .010"?

I believe it can be if results are repeatable

 

[Endyo]

Is seating differences of .003" that important when I am "jumping" the bullet .010"?

Richard, Give Bob a call. I spoke with him today. He said in his testing 0.002" difference may be ok but 0.003" and greater is when he noticed groups opened up.

He suggested I run this test to prove it to myself.

Load 10-rounds at target length.
Load 2 rounds 0.002" under target length
Color bullet with red magic marker
Load 2 rounds 0.002" over target length
Color bullet with black magic marker.
Shoot groups at 200 yards and check results.

 

[dkhunt14]

Is seating differences of .003" that important when I am "jumping" the bullet .010"?

I believe it would be more important when jumping then if you are in the lands 15. ASk Tom how important 2 or 3 thousandth's is at 1000 when jumping. Matt

 

[abersfelderami]

CBTO 3272

Is seating differences of .003" that important when I am "jumping" the bullet .010"?

Yes .003" makes a difference, even more so with hybrid. The picture shows the groupings each one increasing .003"
Berger 200gr Hybrid
jump
CBTO 3.266"
CBTO 3.269"
CBTO 3.272"

 

[snowpro440]

Load 10-rounds at target length.
Load 2 rounds 0.002" under target length
Color bullet with red magic marker
Load 2 rounds 0.002" over target length
Color bullet with black magic marker.
Shoot groups at 200 yards and check results.

 

[CH Luke]

Sold on the value of .003", even more so in the 200 hybrid than the 185 Juggs. Willing to spend the money on the right tools like all those on this thread.

My question is the benefit of the BGC for sorting over the simple Hornady tool that I used (with the callus on the outside of my right thin tip to prove it)? Typically I sort BTO's by .0005 increments with the Hornady comparator. Use the same tool for CBTO with seating and am confident I am within .0005 to .001 max 99% of the time.
It does take high levels of concentration so I wouldn't be opposed to an easier way to do it and maybe save a little time and maybe pickup an improvement in bullet sort tolerance.
Is the BGC worth it?

 

[Jet]

CH Luke

I find the BGC worth it. Sure it's pricey but it helps me eliminate variables. Consistency in all things are critical to shooting small and the BCG helps me eliminate one of them.

Good Shooting

Rich

 

[noload]

I've owned and used Bob's tool (BGC) many years now. The BGC will allow you to sort bullets so they give the best chance of setting the seating lenght the same on all the sorted bullets. I find your seating dimension will be within .001" on the sorted bullets.( Depends how close you sort the bullets) If you do not sort with his tool and seat bullets you will have a few out of spec, some loads will be .002-003" out of spec. So you could load 60 loads and find 50 that are within spec. I do not Use the BGC for every match because I think the Berger Bullets I use are very good . I do not know how consistent the dimensions are on other bullet makers though. So you can sort them before you load or sort after and use the out of spec for foulers.

 

[Endyo]

CBTO 3272

Yes .003" makes a difference, even more so with hybrid. The picture shows the groupings each one increasing .003"
Berger 200gr Hybrid
jump
CBTO 3.266"
CBTO 3.269"
CBTO 3.272"

Thank you for the test. Far more valuable than theoretical discussion.

I had just loaded up a similar seating test this evening, but you beat me to it! Hopefully my results mirror yours.

I'm very close to purchasing a BGC.

 

[Kings X]

Ok, I'll look into it. But kind of makes all my previous load test useless.

 

[dmoran]

CH Luke

For myself, BtO (my A to E reference) qualifications I feel is the most important to equalizing a Lot of bullets to where they come in contact with the lands.
BtSS (my A to F reference) qualification is to equalize my seating depth measurements, by where the bullets will contact the seating die's stem.
My tare reference (same as BGC) gives me the equality of both the above to each other. From which to have equal bullets all 3 aspects have to be equal, to get perfect matching bullets, to the seater diameter. Then from that point ahead to the tip (my F to G reference) can be equalized by meplate trimming and/or pointing, if need be.

Typically with a good Lot of bullets I see the two can fallow each other linearly quite well. In some instances good enough that only one of the 2 ways need to be used for qualification. Those are the bullets I like, and strive to find, but we can make equal and/or matching bullets, by the above qualifications for any Lot.
Donovan

 

[Kyle Schultz]

... My own way is 3 measurements of qualification for lengths:
1 - from A to E for a first qualification
2 - from A to F for a second qualification
3 - the tare length between E and F is my final qualification of the lengths (which is what the Bob Green Comparator measures)...

Donovan:

How do you measure A to F? Thanks!