CBTO and Lands and different Bullets

[CanMike]

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

 

[John Glidewell]

No. Every bullet will be different and some will be different lot to lot.

 

[Hickswr]

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.

 

[Im2bent]

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

 

[John Glidewell]

Different Ogives engage in different places. Secant, Tangent, Hybrid and now the newer long range hybrid designs. They will all be different CBTO.

 

[CanMike]

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

 

[lash]

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

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.

 

[RegionRat]

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

Ye Olde Saying about a picture is worth a thousand words.....

Here too, one actual test is worth many words....

 

[CanMike]

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

 

[spife7980]

You can luck into that critical distance matching or not

 

[lash]

View attachment 8779686
You can luck into that critical distance matching or not

I did not know that. Appreciate the information.

 

[Baron23]

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

And exactly how did you measure this, Mike?

 

[CanMike]

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

 

[Rocketmandb]

No. Every bullet will be different and some will be different lot to lot.

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.

 

[Rocketmandb]

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.

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.