Puzzling CBTO Measurement
- Thread starter kaskin
- Start date
Yes, at least one of your measurements is incorrect. Load a dummy round using the 2.194 Cbto measurement, put it in the chamber and see if the bolt closes. I usually take measurements until I have four or five in a row all within a thousand of each other.
Nope, wrong.
Different bullets, different ogjives, different points of contact with the rifling.
Different bullets, different ogjives, different points of contact with the rifling.
No. The shape of the ogjive determines the point of contact. You can’t simply take a Hornady SST, measure the ogjive with a comparator, and assume it’s going to contact the rifling at a certain oal because you have a prior measurement using an ELD-M. A SST has a different shape than an ELD. A long ELD gonna be different than a short ELD. You can’t assume. You have to test fit in your chamber.
Agreed. Had a brain fart moment. I ended up making a dummy round to 2.806 AOL. Chambered it and it had distinct land marks. Called hornady since their book shows 2.810 AOL for ELD-M and without me saying anything else, he asked if it was a Ruger American. Indeed it is. He said they have seen a few Rugers with short throats. His and two other guys in his office had to send them back to get them reamed out by Ruger. So, I will be contacting them and sending this rifle for teaming. I guessed it payed off to ask, do this numbers make sense? Lol 
Perhaps i misunderstood your question I thought you were determining max CBTO, or the distance to the lands. That measurement from base of cartridge to the ogive for any bullet will be the same. You will see a major difference in seating depth of the different style bullets.
My friend, are you saying the distance from the the base of the cartridge to the lands varies? If you think it does, think again. You guys have fun.
I’m saying the distance from the base of the cartridge to where the comparator contacts the ogjive of a bullet touching the lands will vary depending on the bullet.
Hornady modified case is at SAMMI specs, whereas most size with minimal bump on the the shoulder which also gives 2-5 thou difference in base to ogive from SAMMI. Never mind different bullets.
I only use dummy rounds sized to my needs and very minor neck sizing to allow the bullet to be pushed into the case, yet not stick in the lands. Rinse and repeat closing the bolt on the dummy round until I get 3 equal measurements. I will usually use a cleaning rod resting on the bullet as I open the bolt to give it a little extra assistance from the minor land contact without pushing it back into the case.
Option 2 is drill and tap the primer pocket for the Hornady tool with a sized case.
I only use dummy rounds sized to my needs and very minor neck sizing to allow the bullet to be pushed into the case, yet not stick in the lands. Rinse and repeat closing the bolt on the dummy round until I get 3 equal measurements. I will usually use a cleaning rod resting on the bullet as I open the bolt to give it a little extra assistance from the minor land contact without pushing it back into the case.
Option 2 is drill and tap the primer pocket for the Hornady tool with a sized case.
I think what is left out of the OP is what made those lines on the bullets. The lines were not made by the lands and the lines do not depict the distance from the bolt face to the lands. The lines are the point where the comparator sleeve contacted the bullet. The comparator sleeve hole diameter is smaller than the groove diameter and the different shapes of bullet noses will cause the contact point of the sleeve to be at different lengths.
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