Sorting Bullets by BTO, what are we really doing?

[Laguna Freak]

Say I'm getting .75"@100 initially, then seating my .003 increments I notice .018 is .5"@100, .021 is .3" @100, .024 is .1 @100
But notice groups opening up at .027.
This give me a window of accuracy on target of .5 moa over .009" of seating depth if loaded on front side of .024 off the lands.

It seems like you are using the method Erik Cortina promotes. No doubt a very good method.

When I tried it, I got POI’s that were significantly different as I worked that method from say a 0.068 jump to a 0.078 jump even though the 3 or more shots at each jump distance grouped reasonably or sub-moa. As a result I took it one step further by doing a modified ladder type test where I would shoot the most promising seating depths, eg; 1 shot each from say 0.068 to 0.078 at 0.001 increments to identify which cbto lengths grouped closest together. My emphasis is on very low vertical spread as long as the horizontal spread was moa or smaller and not more than 50% greater than the vertical spread. I have developed confidence that this method precisely identifies the full width of a seating depth accuracy node. I will not accept a node that is less than 0.003 wide because it is too narrow to be of substantive benefit in my opinion.

 

[Martin52]

....if these measurements leave you fighting to nail desired CBTO, then you should consider an ogive comparator(a ~BGC as mentioned) to qualify nose shapes before seating.
But you still have to measure each CBTO to know it.

BTO itself means nothing until you determine the source of deviation within it's base length + bearing length + ogive datum. You should consider this truth & not go beyond until you understand this measure.

I use a BGC as step 1 then sort further by projectile base to ogive and also by projectile OAL. Tedious but produces less variance in loaded rounds i.e. cartridge base to ogive measurement and thus assists consistency in the amount of jump.

 

[EdHarley]

When I'm surfing, I try to avoid eddies. But I'm really drawn to this one regarding BTO and BS (Bearing Surface).

I only precision reload for my .308 and primarily use Bergers and SMK's and though I do no competitive shooting, I endeavor to load the best possible cartridges I can to best measure what skill I might have as a marksman. To that end, being schooled with some science and physics, AND being a bit of a perfectionist. . . I'll measure everything and take notes to best understand what to do to get the most consistent cartridges (when one is retires, it's a lot easier to do ).

So, about 3 years back I got a batch of SMK's that have a large difference in BTO's and also in their BSL's. So, I sorted them by BSL (Bearing Surface Length) and selected 20 from each end of the spread to fire and record the results. There was a .034 difference in the BSL's and I wanted to see what kind of difference it makes (if anything). Though my reloading equipment wasn't as good as it is now to give me good consistency for record, the data and what showed on paper did stand out as a significant difference.

Some of the things I saw in my measurements were:
*A strong correlation between BSL and seating depth
*A correlation between BSL and BTO where the difference was close to the same
*A correlation between BSL and the bullet's OAL
*No noticeable correlation to the bullet's weight

Also, I noted a noticeable POI shift between the two sets of 20 bullets.

See pics below.

For a time, I'd sort by BSL, but when I looked at the variance between BTO's and BSL's, it was so little that I just sorted by BTO (if I find difference's in the lot/batch greater than .003). Now though, I don't use a comparator with the diameter of the ogive to sort, but use a comparator with the diameter of my seating stem. Whenever I sort now, this gives me very consistent CBTO measurements (that's right, not the same tool used for sorting).




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How do you assure consistency with the BTO measurement? What device do you use? My Franklin comparator is very inconsistent.

 

[Straightshooter1]

How do you assure consistency with the BTO measurement? What device do you use? My Franklin comparator is very inconsistent.

I use the Sinclair Bullet Sorting Stand (see pics) and a Comparator. To get the best consistency, I drop the bullet into the comparator as vertical as I can from a very short distance then wiggling ever so lightly so the flat surface of the bullet's base is flat to the dial indicator's flat surface. I just takes a few to get a feel for it and it can go rather quickly sorting + or - a zero point (which is a point I determine is at or near a midpoint). After sorting, I'll measure some samples from of each group to record the length of the BTO's for my records.