Bullet comparator

[Sebrock]

So I’m using a Hollands bullet comparator which is probably the most repeatable device I’ve used - I’ve noticed that the ogive datum it uses is .025 (which I believe is similar to what Hornady uses) - for comparison the SAC one I also have is .0255 (which I believe coincides with the SAAMI where the lands begin….maybe…..or much closer to it). The SAC is not as repeatable but gives a lot of variation whereas the Hollands does not. Which is the correct datum to use for BTO and does it even matter as long as you use the same method to sort?

@BartsBullets - didn’t want to ask you this on the ST Facebook group but was hoping to get your thoughts.

 

[762plinker]

it would be nice if your seating die, your chamber and your comparitor all had the same dia, but sadly it aint likely and you did not say what bullet class, 6mm 25 cal or 264

 

[Sebrock]

it would be nice if your seating die, your chamber and your comparitor all had the same dia, but sadly it aint likely

I know. Right. I also played with measurements and finding jam (by slowly seating a bullet with the bolt) with both methods and the only thing that is indicative of Jam was large differences in OAL minus or plus the Holland’s BTO measurement. But when comparing the two comparators there was no rhyme or reason as to what I would get.

 

[762plinker]

I know. Right. I also played with measurements and finding jam (by slowly seating a bullet with the bolt) with both methods and the only thing that is indicative of Jam was large differences in OAL minus or plus the Holland’s BTO measurement. But when comparing the two comparators there was no rhyme or reason as to what I would get.

polish the bullet and get a magnifying glass. you should be able to see all four lands as they touch.

 

[Sebrock]

polish the bullet and get a magnifying glass. you should be able to see all four lands as they touch.

I’ll do that and compare datum’s. I use an Accuracy One bullet seating gauge so it measures from the shoulder to ogive. What I notice is that neither method is comparative of how a bullet will seat. Meaning the SAC can show +.0015 between two bullets and the Hollands is 0 and the seating measurement is 0. But there are times it is the reverse. I can also tell you that when I sort by OAL and then used Holland’s to sort by BTO - the bullets are all very similar and consistent (and you see what Bryan Litz has said, longer bullets, shorter BTO) but my guess as far as distance to lands is that both comparators get you within your node but it’s almost impossible to get a perfect measurement. So like I said in the beginning maybe it doesn’t matter as long as whatever you use is consistent?

 

[mikecr]

So what attribute are you hoping to derive from BTO?
Ogive radius, bearing length, base length? For BC?
How will it matter?

 

[Sebrock]

So what attribute are you hoping to derive from BTO?
Ogive radius, bearing length, base length? For BC?
How will it matter?

I do it for BC and happy with my process - OAL and BTO as a proxy for bearing lenght, combined give a great idea of bullet shape differences and it’s easy to sort that way. I was hoping that I could use BTO to get super consistent distance to the lands and if it made sense to alter the seating but upon my experimentation and reading through some of the bullet comparator threads, it seems like a futile effort as there are always going to be small differences but was wondering if one comparator was better than the other for this aspect. Also in general is one comparator more indicative of bullet shape, bearing length more than another? The SAC more sensitive but not as repeatable by far.

 

[Ned Ludd]

Although it is desirable for the comparator insert to seat as close as possible to the exact point on the bullet ogive that first touches the rifling, we don't always have that luxury. Because we can take measurements directly from loaded rounds, using a slightly smaller comparator insert diameter that seat just a bit farther out on the bullet ogive will work just fine as long as you use that same insert for all measurements. In other words, the relative measurments will be the same, but from a difference reference point. You can still produce loaded rounds with very tight CBTO tolerances using a comparator insert with a slightly smaller diameter hole.

 

[Sebrock]

Although it is desirable for the comparator insert to seat as close as possible to the exact point on the bullet ogive that first touches the rifling, we don't always have that luxury. Because we can take measurements directly from loaded rounds, using a slightly smaller comparator insert diameter that seat just a bit farther out on the bullet ogive will work just fine as long as you use that same insert for all measurements. In other words, the relative measurments will be the same, but from a difference reference point. You can still produce loaded rounds with very tight CBTO tolerances using a comparator insert with a slightly smaller diameter hole.

Thank you @Ned Ludd and I suppose that being the case, I’ll take the repeatability of the Holland’s. Very helpful.

 

[BartsBullets]

So I’m using a Hollands bullet comparator which is probably the most repeatable device I’ve used - I’ve noticed that the ogive datum it uses is .025 (which I believe is similar to what Hornady uses) - for comparison the SAC one I also have is .0255 (which I believe coincides with the SAAMI where the lands begin….maybe…..or much closer to it). The SAC is not as repeatable but gives a lot of variation whereas the Hollands does not. Which is the correct datum to use for BTO and does it even matter as long as you use the same method to sort?

@BartsBullets - didn’t want to ask you this on the ST Facebook group but was hoping to get your thoughts.

As far as which one to use I don’t have a clue. If you’re measuring the difference in bearing surface it’s a waste of time with CUSTOM bullets.

Any variation in bearing surface will be straightened out by your bullet seater.

Bart

 

[mikecr]

Bearing and base length won't matter to seating.
The tool datum won't matter to either CBTO or COAL.
If your seating testing leads to x.xxx as best, as measured by any common tool, then you just repeat that.
However, the nose shape (ogive radius) affects tool datum.
So for valid BTO or CBTO, you need to 1st qualify the nose shape.

You can do this with a Bob Green Comparator (BGC), or separate.
This is resolving a comparative difference between seating stem datum to proposed land contact datum, from bullet to bullet

If you were really committed to seeing a difference, you could separate measurements to estimate drag, and derive BC from each individual bullet. This is weight, diameter, OAL, end diameter, base length and angle, bearing length, nose length, ogive radius, and meplat diameter.
Of these, the biggest impact(to BC) in variance is likely meplat diameter.
Yet, I see no shiny toys to measure that..

I haven't read of anyone doing all this in a long time. Henry Childs was the last I knew of,, [a BRC forum member posting over ~20yrs ago]. I suggest, and I'm sure Henry would that half measures (or less) are a waste of time to do, and actually acting on them is outright foolish.
You're better to just get good bullets in a big batch.

Ideally, we could procure bullets pre-qualified like this. Someone with a Keyence laser comparator that could get instant readings, loading into spreadsheet fields, for BC estimation -per bullet.
They're VERY expensive, but If I were a big bullet maker I would do this.
Provide special offerings that match (as declared) in estimated BC.
I'm thinking those bullets would need to cost an extra buck each. !!

 

[Sebrock]

I sort by feel.

 

[Sebrock]

Bearing and base length won't matter to seating.
The tool datum won't matter to either CBTO or COAL.
If your seating testing leads to x.xxx as best, as measured by any common tool, then you just repeat that.
However, the nose shape (ogive radius) affects tool datum.
So for valid BTO or CBTO, you need to 1st qualify the nose shape.

You can do this with a Bob Green Comparator (BGC), or separate.
This is resolving a comparative difference between seating stem datum to proposed land contact datum, from bullet to bullet

If you were really committed to seeing a difference, you could separate measurements to estimate drag, and derive BC from each individual bullet. This is weight, diameter, OAL, end diameter, base length and angle, bearing length, nose length, ogive radius, and meplat diameter.
Of these, the biggest impact(to BC) in variance is likely meplat diameter.
Yet, I see no shiny toys to measure that..

I haven't read of anyone doing all this in a long time. Henry Childs was the last I knew of,, [a BRC forum member posting over ~20yrs ago]. I suggest, and I'm sure Henry would that half measures (or less) are a waste of time to do, and actually acting on them is outright foolish.
You're better to just get good bullets in a big batch.

Ideally, we could procure bullets pre-qualified like this. Someone with a Keyence laser comparator that could get instant readings, loading into spreadsheet fields, for BC estimation -per bullet.
They're VERY expensive, but If I were a big bullet maker I would do this.
Provide special offerings that match (as declared) in estimated BC.
I'm thinking those bullets would need to cost an extra buck each. !!

Yes - I think my mistake was mixing the seating and the sorting for BCs sake. I was able to speak to the maker of my comparator and he shared much of the advice I have been given here.

 

[dcali]

Good bullets should all be very close to the same no matter what the diameter is. If you need to sort them, I would suggest getting better bullets, or not sorting them and just using them in guns where you don't care so much about accuracy. Either way, sorting by BTO isn't a very productive use of time, and you're better off sorting by OAL anyhow.

 

[dcali]

If you were really committed to seeing a difference, you could separate measurements to estimate drag, and derive BC from each individual bullet. This is weight, diameter, OAL, end diameter, base length and angle, bearing length, nose length, ogive radius, and meplat diameter.
Of these, the biggest impact(to BC) in variance is likely meplat diameter.
Yet, I see no shiny toys to measure that..

I think this is the right concept for sorting long range bullets. Practically, weight, diameter, boattail dimensions, and ogive radius don't matter because they should be very consistent- their variation's impact on BC is negligible. Overall length, bearing surface length, nose length, and meplat diameter, are all related and can be accounted for just by measuring OAL.

One side note about meplats:

Meplat diameter and its impact on BC often misunderstood. The reason it matters is that it defines the shape of the bullet, not because the diameter by itself is particularly impactful. You can't shrink the meplat without altering the ogive or making the nose longer. Those are the things that impact BC. Similarly, people often assume that when you point a bullet, the aim is to close up the meplat. It's not. The goal is to transform the tip of the bullet into a shape that is closer to the minimum drag shape. This often involves closing up the meplat, but that isn't what we're trying to do. We're trying to change the shape of the nose tip.

 

[Sebrock]

I think this is the right concept for sorting long range bullets. Practically, weight, diameter, boattail dimensions, and ogive radius don't matter because they should be very consistent- their variation's impact on BC is negligible. Overall length, bearing surface length, nose length, and meplat diameter, are all related and can be accounted for just by measuring OAL.

One side note about meplats:

Meplat diameter and its impact on BC often misunderstood. The reason it matters is that it defines the shape of the bullet, not because the diameter by itself is particularly impactful. You can't shrink the meplat without altering the ogive or making the nose longer. Those are the things that impact BC. Similarly, people often assume that when you point a bullet, the aim is to close up the meplat. It's not. The goal is to transform the tip of the bullet into a shape that is closer to the minimum drag shape. This often involves closing up the meplat, but that isn't what we're trying to do. We're trying to change the shape of the nose tip.

So I agree with everything you are saying and I use only high quality match bullets - they vary very little - only reason I use BTO is that a difference there catches the differences in shape with higher resolution given that for the Berger bullets and ELD-Ms I use, the OAL varies ever so slightly (that is my understanding an how it was explained to me). But yes - I am measuring for bullet shape differences - and this whole exercise I have been through has helped me realize thats what Im doing and seeing the relationships you mention - it has been very educational because to be honest, I didnt really know. All Im doing now is looking for the outliers to sort out, or figure out differences when I switch lots. Interestingly - and as you alluded to - when I sort this way, there is maybe one or two in a hundred, very impressed at the uniformity Im finding. And yes - this is only for 700+ yards on out. So happy that there are bullet makers here because this is the kind of advice Im looking for so much appreciated.

 

[mikecr]

It's just important to recognize that summation measurements may not be appropriate to act on.
An attribute that reduces BC here can lead to another that increases BC over there, providing a very good bullet overall.
So if you just cull based on the first general difference, you could be culling your best bullets. You could be increasing your actual BC variance in lot, all with a notion that your action is apparently making things consistent.

I've often felt that consistency cannot be credited without actual understanding.

 

[Sebrock]

It's just important to recognize that summation measurements may not be appropriate to act on.
An attribute that reduces BC here can lead to another that increases BC over there, providing a very good bullet overall.
So if you just cull based on the first general difference, you could be culling your best bullets. You could be increasing your actual BC variance in lot, all with a notion that your action is apparently making things consistent.

I've often felt that consistency cannot be credited without actual understanding.

Thats why I'm down to buying good bullets and culling out the one or two bad bullets in a batch. With my current comparator, almost every bullet in a batch is within .001 BTO, it's just every so often there is one or two that are significantly different. In those, the length will be different too (obviously) - it almost appears that they are from a different lot. Cant hurt to take those out, no? Everything else I am not sorting.