Mixing Bullet Lots by Length

[Straightshooter1]

I'm really not trying to be a jerk, but how can sorting bullets by BTO or BS help improve seating depth? That is an honest and fair question. Obtaining uniform seating depth requires the exact same distance on bullets between the two critical contact points, i.e. where the seating die stem contacts the nose to push the bullet down into the case neck, and where the comparator insert seats on the bullet ogive when we measure CBTO. Both of those critical points are located on the bullet ogive and thus fall outside either BTO or BS measurements. In other words, sorting bullets by BS or BTO are unlikely to improve uniformity of the nose length region, of which they lie outside. Thus, I don't think it likely that sorting bullets by BTO or BS will be effective at generating more consistent seating depth.

In fact, Bob Green and others make Comparator tools that specifically sort bullets between those two critical contact points on the bullet ogive. Ideally, the use of such a tool would be even better than sorting bullets by OAL, which admittedly also takes bullet regions outside those critical points into account. For that reason, I also view sorting bullets by OAL as a "poor man's" Bob Green Comparator. However, because the major source of bullet length variance typically resides in the nose section, sorting by OAL does help to improve the consistency of seating depth. I don't see how sorting by BTO or BS will make any contribution in that regard. As I alluded to earlier, many people that sort bullets by BTO or BS do so for very specific reasons that result from having the exact same length of BS gripped in the neck at a given seating depth, which are different reasons from promoting uniform seating depth. To be clear, I'm not knocking sorting bullets by BTO, BS, or whatever other parameter anyone wishes to use. In fact, any sorting procedure might be a possible improvement, and is unlikely to ever make precision worse, even if it doesn't improve anything. I'm merely questioning whether sorting by BS or BTO will help promote uniform seating depth.

As you say, "the major source of bullet length variance typically resides in the nose section", so by sorting either by BTO or BS, one is mitigating a "major" part of the variance one has in the OAL's. An observation of mine when I did my little BS experiment just a few years back on some 168 SMK's, there wasn't really any correlation between the BTO or BS variances and the OAL variances (e.g. I could have bullets with the same BS or BTO but each had a every different OAL or vica versa).

Just like all the other bullet segments, there will be variance from base to a seating stem's contact point. Even though a seating stem will have a contact point above a comparator's ojive contact point (call it BTSS for Base To Seating Stem, if you like), there's less of a variance from the BTSS than OAL due to the nose having the greater variance. Likewise, there's not any direct correlation I could see between the BTO variances and the BTSS variances. But the variances are smaller than between BTO and OAL.

In terms of promoting uniform seating depths: sorting by BS just didn't really do it, though it did make a difference on paper when the variances were large (like in my little experiment where the BS variance difference was ~.034). Sorting by BTO, in my experience, does promote more uniform seating depths. Better yet, sorting by BTSS has been giving me the most consistent seating depths for boat tail bullets, which I haven't really quantified in comparing BTO and BTSS measurements.

lol . . . sounds like something I'll have to do when I'm feeling bored again.

 

[dcali]

First, to answer the actual question, I would not mix lots, even if the bullets look the same. You might get away with it. It *should* be ok. But weird stuff like differing jacket hardness can perhaps be significant.

As for OAL or BTO? Decent bullets shouldn't vary much in BTO. Depending on the specific bullet, it may easier or harder to measure anyhow - the slight angle of the ogive at the point of measurement, combined with the radius/corner of the comparator insert will give even a skilled shooter some variation in measurement. It's easy to tip the bullet a little, dig in to the ogive, or otherwise flub the measurement. Measuring OAL on the other hand is very easy, and given the much larger variation in OAL, should be more meaningful.

I agree with Ned - the nose is the part of the bullet with the most variation, and contributes a large portion of the BC. A small difference in length will make a small but real change in BC. If you're going to sort, I'd do it by OAL. To his point on consistent seating, I'm not sure there's a perfect answer to that. I've tried using barrel stubs as a comparator, which in theory should work really well, but I didn't have any luck getting consistent measurements. Others may have better ideas.

 

[dcali]

I don't understand how a tiny difference in oal perhaps .001 or .002 ( a human hair) at the meplat can make any significant difference at any distance on paper or effect BC in a way that's observable, on the other hand seating depth variances of .001 Will show at 1000 yards as well as .1gr powder variances and bushing size can make a huge difference on paper and that I can see but OAL ??? Im not so sure...

It's not the size of the meplat. That's a bit of a misconception. Here's a probably too long explanation:

If you look at research going way back to the 30s at least, engineers looked at various nose shapes and meplat types trying to figure out their drag properties. They looked at (among other things), nose length, meplat size, and headshape parameter (the ratio of a tangent ogive to the bullet's actual ogive radius - 1.0 would be a tangent. 0.5 would be an aggressive secant). Headshape parameter is also referred to as Rt/R (Bryan Litz's books use that notation).

So how did they go about it? They would take a fixed nose length, and vary the headshape parameter, for example. But when you do that you also *must* vary the meplat diameter, or the length would have to change too. So these three factors combine in ways you can't separate. It's just plain old geometry.

So that's the context of the "smaller meplat is better". It's not true in a vacuum - it's a part of the bullets nose's overall geometry. So when point a bullet, you are not changing the meplat diameter in this sense. What you are really doing is changing the *shape* of the tip (as geometry necessitates - the ogive radius and nose length are already set in metal), which can also have a small but noticeable impact on drag. The precise shape of the optimal tip has also been researched fairly extensively. It's not going to be the same for every bullet, and it's not a flat meplat. It's more of a cross between a cone and a hemispehere.

So long story short, you're not just changing the meplat by .002. You're reshaping the point.

And for what it's worth, small variations in overall length (.010-.020 or so, very roughly speaking) do have a small but measurable impact on BC. The easiest way to make a bullet with a high BC is to make it longer. Like much in life, there are ballistic tradeoffs, but that method works well until it doesn't.

It's all this "small but measurable" stuff that gets us into trouble. At some point we make it "small and negligible", and none of us have the sense to stop.

 

[Killick]

If i ever get time I would read a lot of books, would you say he's making bad bullets better? or more consistent and perhaps at that point one could sort oal and have it provide a more useful information whereas I measure production bullets BBTO only looking for outliers. Lately I've used Vapor Trails that are more consistent than my tools at any datum so very little sorting and more shooting.

Generally, yes, bad bullets better. And yes, more consistent as far as BC. CBTO would be a more useful parameter than BBTO per Litz. The better the bullet the lower the percent of BC improvement. Now I've found Bergers to be so consistent as to make sorting them irrelevant...except for length. Even then I have sorted different lots recently and they all seem to vary between the same .012" with the lion share being in the middle .004". This just leads me to believe that mixing the lots by length is probably doable.

 

[triggerdog]

Do all 3 , by weight and length and...

 

[Killick]

Do all 3 , by weight and length and...

Wasted effort.

 

[triggerdog]

Forgot the 3ed one....

 

[triggerdog]

I take a piece of 100 grit sandpaper and make all my sierra bullets the same exact length, takes a while but proof is on paper....

 

[Killick]

Old school, nice.