OBL at the lands

[greenwood]

I am somewhat of a reloading novice. A question for the seasoned and knowledgeable. Does OBL measured at the lands for a specific rifle remain the same for all standard bullets regardless of weight, size and configuration?
My logic is that the distance from the base of the cartridge to the lands (beginning of the rifling) is always the same for a specific rifle. The chamber was reamed to a specific depth when the rifle was built and the chamber length does not change unless the lands are eroded or the bolt is changed. Thus, the OBL measured to the lands, the distance from the base of the cartridge to the ogive (where the bullet diameter is the same as the grooves and the bullet is constrained by the lands) is fixed . Regardless of the weight or shape of a specific bullet seated in the cartridge neck, this distance, the OBL measured to the lands, will remain the same. However, the overall length of the cartridge + bullet measured to the bullet tip with the bullet measured at the lands (COAL) will change with bullet configuration, design and weight, but not the OBL measured at the lands -the OBL at the lands is static.

 

[ar10ar15man]

what is an obl ??/

 

[greenwood]

you may call it an aol, etc. It is the distance from the base of the cartridge to the ogive touching the lands. Measured with a comporator.

 

[Shynloco]

greenwood,
No offense meant, but I frankly don't follow what you are getting at. Given that we know every rifle is different and that every bullet in that specific rifle very likely wil have a different ojive measurement due to configuration of the bullet that is different by each manufactures ' design of bullets, AND each rifle will also likely require a different depth seating for best accuracy based upon each different bullet tested, theres NO WAY in my way of thinking that your stated question for ALL bullets (for each specific rifle) would have the same OBL nor probably the same OAL due to the seating issue previously mentioned. Am I missing something here? Thx

Alex

 

[greenwood]

Perhaps I can explain my question and logic with an example.
If we seated a 100 grain tanget ogive bullet touching the lands and a 100 grain secant ogive bullet touching the lands would the measurement from the cartridge base to the bullet ogive be the same for both bullets?
Further, if you seated a 125 grain secant ogive and 125 tanget ogive bullet touching the lands would all four of the seated cartridges have the same OBL measurement? (OBL I believe means "ogive base length".) Each bullet may be seated at a different depth into the neck and the area above the ogive may protrude into the barrel more or less due to the design of the bullet, but the ogive will be resting against the lands in all cases. But because the measurement is made from the cartridge base to the ogive, which is against the lands, the OBL distance should be the same as distance from the cartridge base to the lands and the length is constant. Thus, the OBL will be constant. Is my logic flawed??

 

[Nomad47]

Yes, it is flawed. And the term you want to use is CBTO (cartridge base to ogive).

For example, the lands on a 6mm barrel are either .236 or .237, but the I.D. of my 6mm comparator is .234 (and your comparator is likely different). So it is contacting the bullet at a point inside the lands and depending on the taper of the bullet, this will vary from bullet to bullet.

 

[Killshot]

If we seated a 100 grain tanget ogive bullet touching the lands and a 100 grain secant ogive bullet touching the lands would the measurement from the cartridge base to the bullet ogive be the same for both bullets?

No, it would not be the same.
The datum on the ogive will not be in the same location on those two bullets.
Nor would it be the same on a same-brand 105gr VLD and a 90gr VLD.
Shynloco also notes that our measurement-taking is sometimes not a true record in that where we measure the datum on the bullet is not the actual location of land-engagement in the barrel.

 

[Steve Blair]

Here are the Sinclair measurements taken from a Hornady tool and a variety of bullets in my .308 tactical rifle. They range from 155 to 210 grains, are Lapua, Sierra, Berger and Hornady and all are just touching the leade. Not the same.


2.038
2.032
2.027
2.040
2.031
2.040
2.023
2.014
2.050
2.015
2.048

 

[kelbro]

It seems like it would be the same if your comparator matched your barrel.