CBTO variations??

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Kpuck08

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Should your CBTO measurement differ from bullet to bullet? I realize your overall length will, but I would have thought the distance to the lands should be the same regardless of bullet choice.

6 ARC Proof CF barrel, Hornady OAL gauge, with 87Vmax my CBTO is 1.670 and with the 90 gr Sierra Game changer it is 1.725. Am I reading into this wrong or am I miss using the Hornady gauge?
 
Every make, brand, size, and bullet model will have it's own base to ogive measurement, even different lot numbers of the same bullet can & do vary.

Important to always verify specs when buying new bullets, especially when you load close or into the lands.
 
The cbto isn't the only factor in seating bullets, bearing surface and neck tension are important. So a sweet spot(cbto) with a brand/type of bullet may not be useful with other bullets.
 
So in my head I was assuming the diameter of my Hornady bullet comparator was the same diameter as the lands. The different readings make sense if the Hornady tool isn’t necessarily measuring where the bullet hits the rifling.

I must have a short throat on this one, at 1.670 CBTO with the 87vmax, I am touching the lands at hornadys recommended COAL of 2.140.
 
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Depending on action type it may be irrelevant. An AR15 for example, seating depth is limited by inside magazine dimensions. I have to seat them so they feed properly & the jump to the lands simply isn't a concern.

A good bolt action (or others) can be a different story though & you may see gains by tweaking seating depth.
 
Originally Posted By: Stu FarishDepending on action type it may be irrelevant. An AR15 for example, seating depth is limited by inside magazine dimensions. I have to seat them so they feed properly & the jump to the lands simply isn't a concern.
Not necessarily an accurate blanket statement.....

I have two AR barrels that I can touch the lands and stay within the confines of an ASC mag.
 
You are correct a Hornady comparator will not get you an actual measurement. A good skill is to be able to find touch(cartridge length where the bullet begins to contact the rifling). Once you have a dummy round at just touch, the comparator can be used to know how far you are seating away from touch(into lands or further off) while testing. For hunting rounds, I work from there, seating deeper into the case until best seating depth is found. Often manufacturers will have general guidelines for seating, Barnes for example recommends seating 0.050 off lands and test increasing 0.060, 0.070. Touching length will increase in length as the rifle is shot.
 
That's a great point:

You should never take anything for granted. But for mag fed rifles, if you're seated at the max it will feed reliably, that's as far out as you're going to get.

One thing I've often done when I had a new rifle & found factory ammo that it shot well, was adjust my seating die off that ammo as a starting point. I've found that it's generally worked well for me.
 
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