CBTO and Lands and different Bullets

C

CanMike

Private
Minuteman
Sep 2, 2025
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29
Ontario, Canada
#1
I may be overthinking this.

So if I use one bullet let’s say the 140 ElDM to find the lands and get a CBTO measurement there.

Now if I want to seat 0.020 off that point I just adjust down my seating die etc. pretty simple.

Now if I have second bullet let’s say the 153 ATIP can I use the above CBTO of lands and adjust accordingly.

My understanding is that the OGIVE is the point the bullet engages the rifling. So that theoretically should be same across all brands of bullets? Obviously at a different point on each type of bullet but CBTO to lands should be same across all bullets?

Thanks
Mike
 
#3
Bearing surface lengths of different bullets varies greatly.

Just picture a 95gr v-max bullet vs a 153gr a-tip. They will start to engage the rifling at vastly different CBTO lengths.
 
#4
I was under the impression that the contact point/diameter is what it is and that length will be the same, that being said different bullet different seating depth to get that dimension
 
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#6
Okay so went back and redid land touching with the 153 ATips. Was exactly the same CBTO as the 144 lapua long range target. 2.235 to touch lands. Within a couple thousandths which is well within error of measuring.

So both bullets seated at 2.220 give me about 15-17 jump.

Mike
 
#7
CanMike said:
Okay so went back and redid land touching with the 153 ATips. Was exactly the same CBTO as the 144 lapua long range target. 2.235 to touch lands. Within a couple thousandths which is well within error of measuring.

So both bullets seated at 2.220 give me about 15-17 jump.

Mike
Click to expand...
What some are missing is that, as long as your seating die and BTO measuring die are the same diameter as your lands, I.e. to spec., then this will be true. At the ogive.
 
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#8
CanMike said:
Okay so went back and redid land touching with the 153 ATips. Was exactly the same CBTO as the 144 lapua long range target. 2.235 to touch lands. Within a couple thousandths which is well within error of measuring.

So both bullets seated at 2.220 give me about 15-17 jump.

Mike
Click to expand...
Ye Olde Saying about a picture is worth a thousand words.....

Here too, one actual test is worth many words....
 
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#9
Loaded and ready for Range Testing Powder charges. The A Tips are long. Testing a few of them just to get velocity / powder correlation.

Both loaded to 2.220

Mike
 

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#12
CanMike said:
Okay so went back and redid land touching with the 153 ATips. Was exactly the same CBTO as the 144 lapua long range target. 2.235 to touch lands. Within a couple thousandths which is well within error of measuring.

So both bullets seated at 2.220 give me about 15-17 jump.

Mike
Click to expand...
And exactly how did you measure this, Mike?
 
#13
Hornady Comparator. And their OAL Gauge with modified case.

The Lapua I also gently crimped a once flared case and inserted a bullet so there was moderate friction and put JB weld inside the neck and inserted and closed bolt. Let sit overnight.

The hornady gauge is close to that measurement but requires averaging a few to get a ball park.

All these measurements are not 100% accurate. But I guess if you do the same way it’s comparable even though it’s not absolutely accurate.

Mike
 
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#14
John Glidewell said:
No. Every bullet will be different and some will be different lot to lot.
Click to expand...

I recently had a 20 thousandths variance on bullets from the same lot. Ran out of one box during loading, opened another from the same lot, and my AMP Press suddenly started engaging earlier and the terminal force was higher. I pulled one of the original bullets and compared it to a couple of the new ones - the bearing surface was 20 thou longer. And these bullets were from one of the top manufacturers - something changed in their process during that lot.

So, it's not necessarily just different bullets or different lots.
 
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#15
CanMike said:
I also gently crimped a once flared case and inserted a bullet so there was moderate friction and put JB weld inside the neck and inserted and closed bolt. Let sit overnight.
Click to expand...
This is my go-to method now. Though I use blue Loctite. Here's why:

- In the small chance that some gets into your chamber, the heat of combustion of the first round will deal with it.
- It reaches effective cure very quickly
- I take the dummy round used for this, mark it with the round count and date, and use it as a definitive reference. You can't get that from the Comparator.
 
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#18
Rocketmandb said:
... I pulled one of the original bullets and compared it to a couple of the new ones - the bearing surface was 20 thou longer. And these bullets were from one of the top manufacturers - something changed in their process during that lot.
...
Click to expand...
Sierra bullets perhaps?

I had a batch of Sierra MatchKings 155gr 2156 thqt had 3 distinct bullet BTO ranges in the same box. I suspect that they dump bullets from at least 3 different machines into the same batch.
 
#19
Rocketmandb said:
I recently had a 20 thousandths variance on bullets from the same lot. Ran out of one box during loading, opened another from the same lot, and my AMP Press suddenly started engaging earlier and the terminal force was higher. I pulled one of the original bullets and compared it to a couple of the new ones - the bearing surface was 20 thou longer. And these bullets were from one of the top manufacturers - something changed in their process during that lot.

So, it's not necessarily just different bullets or different lots.
Click to expand...
Hornady and Sierra are not top manufacturers
 
#22
CanMike said:
I may be overthinking this.

So if I use one bullet let’s say the 140 ElDM to find the lands and get a CBTO measurement there.

Now if I want to seat 0.020 off that point I just adjust down my seating die etc. pretty simple.

Now if I have second bullet let’s say the 153 ATIP can I use the above CBTO of lands and adjust accordingly.

My understanding is that the OGIVE is the point the bullet engages the rifling. So that theoretically should be same across all brands of bullets? Obviously at a different point on each type of bullet but CBTO to lands should be same across all bullets?

Thanks
Mike
Click to expand...
this v v v v v v v
"No. Every bullet will be different and some will be different lot to lot."

Essentially, every new/different bullet (even the same bullet from a different lot) you'll need to measure bullet's distance to the lands for your starting point in developing a load for it. If you're seating off the lands, you really don't have to be precise for the measurement. . . just so you have a good idea where the lands is in relations to the bullet's BTO. That distance to the lands will change as you change seating depth to find what works best. After finding a starting point (a distance from the lands), you can ignore distance from the lands as you go for a seating depth that work well and keep that seating depth regardless of throat erosion making the jump longer. Jump is not near as important as seating depth.

You might want to keep track of throat erosion. If you do, use a single dedicated bullet to use repeatedly in measuring its distance to the lands to avoid any variance in bullet dimension from one bullet to another or from one lot to another.
 
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#23
If you want a tool, make a comparator insert that is exactly bore diameter. Then, using a reamer, cut a chamfer that is the same as the chamber lead angle. This will mimic the chamber lead allowing an apples to apples comparison between different ogives.
 
#24
Once again, be very careful if using comparators/insers from different manufacturers in the same measurement process. The insert's bore diameter from one manufacturer in all likelihood is NOT the same diameter as that from another manufacturer. I have seen as much as 0.004" difference in diameter for a given caliber between different manufacturers.

For years (if not decades), I had assumed that the bore diameter for all manufacturer's inserts was "standard". I didn't really ever have any reason to think otherwise. They are not. Each manufacturer bores their insert diameters to whatever size they feel is best.

I finally standardized on Short Action Customs comparators and inserts. IIRC, they bore their inserts to -0.001" under bullet diameter. I like the fact that the inserts and the bodies are stainless, as opposed to aluminum.

0.004" variation may not sound like much, but it equates to a lot of distance difference as to where the comparator (insert) finally "jams" (for lack of a better term/description) on the ogive.

Having said all of the above, this does not mean one comparator/insert brand is "better" than another. As long as you make your measurements with one comparator/insert, you should be just fine. But, it also means that exchanging and comparing absolute CBTO measurement values with someone else isn't valid unless you know for a fact they are using the same brand of comparator/insert with exactly the same bore diameter.
 
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