CBTO measurement question

[waspocrew]

Would you trust the Hornady tool for the distance to the lands measurement, or do you use a different method?

I use the Hornady comparator and feel it works well for its purpose. The tough thing is trying to be consistent with it - I measure about 5-10 times and get an average.

 

[notthattomsimonson]

https://bergerbullets.com/wp-content/uploads/2013/03/COAL.pdf

Some reading with illustrations for the OP.

Very helpful, thank you.

 

[EdP]

I trust the Hornady tool when used as I described, similar to what waspocrew said above.

 

[jfk69]

Would you trust the Hornady tool for the distance to the lands measurement, or do you use a different method?

I’ve trusted the Hornady tool for 20+ years to do exactly that.

 

[JFK]

OP, I never load Barnes close to the lands and don’t recommend you do either. They will build pressure much more quickly. First and only time I’ve popped primers was a Barnes too close to the lands that was within suggested charge weight. Barnes suggested COL’s have usually worked well for me. .090 jump isn’t unheard of and I’ve had super accurate loads that were .120 off. Jump isn’t a bad thing. It creates more forgiving ammo.

If you do want to find distance to lands, I seat a bullet purposely long in a dummie round, sharpie the bullet. Then just make very small incremental reductions in COL until it chambers with subtle land marks on the bullet. Bolt close should be easy. If it closes but with increased resistance you are jamming the bullet.

Or don’t do any of this, jump them, and still have accurate, forgiving ammo without the hassle.

 

[wind gypsy]

Would you trust the Hornady tool for the distance to the lands measurement, or do you use a different method?

It’s close enough. You’ll get a feel for it eventually to know when you’re starting to get resistance from the lands.

If you’re loading Barnes 0.050” off the lands, it really shouldn’t matter if your measurement is 5 or 10 thou off.

 

[sun627]

Thanks for this explanation, it makes sense to me now. I was having trouble with a couple of the bullets sticking in the hornady modified case, which made it difficult to know if I was "kissing" the lands, or had to push harder to get the bullet to slide in the case, so that was part of my variability. The long ogive profile of the ttsx probably made it stick sooner in the comparator base, hence the longer reading. I wasn't thinking about how the difference between the diameter of the comparator and my actual lands would affect the measurement.

One related question though. Barnes says to load the 150 gr TSX to a COAL of 3.27. Based on the measurements I did, that would be a 90 thousandths jump, which is way longer than I would have started. But now I feel like I should ignore the CBTO measurements for the reasons mentioned above and just go with the Barnes COAL?

Factory ammos are quite conservative in the seating depth so it won't easily get jammed

Measure CBTO only makes sense if you hand load. Then measure where the land is in your chamber then back calculate the CBTO based on the jump you want.