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Since i'm new to this Forum, i'm not sure how technical to get but here goes.
Building a .260 AI and need to send in a dummy round. I'm working with two differant bullets...Sierra 142grMK and a Berger VLD 140gr, the ogives of these two bullets are differant by about .060" but am trying to find a common ground for COAL. The Bearing Surface of the Sierra Is also Shorter than that of the berger but the question is at what point of the bullet diameter does it contact the rifeling. Does it contact the rifling at the end of the ogive (curve)? Is there a standard at which can be calculated...ie. does a .224 bullet contact at .216 of the bullet dia...does a .243 contact the rifling @ .235, does a .264 contact @ .256??? Just wondering because then I can set my caliper to that dia insert the bullet twist and make a mark on the side, at that point I can measure from the base of the bearing surface to that dia and be consistent with my contact point in the barrel. Any constructive help would be appreciated.
 

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The contact point that each bullet will contact the lands at the ogive will be different for each bullet as you're aware, therefore, no 'standard' dimension exists for what you want to do, in this or any other caliber.

It would be best IMHO, to set up your dummy round for the primary bullet you want to use, and just plan to alter the seating depth for the secondary bullet in order to just kiss the lands.

The amount of bullet protruding into the case using your secondary bullet should be negligable, as we're only talking a few 'thou here, and should not really affect anything, and still allow you to reach or kiss the lands with both bullets.

Make sense? :)
 

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just a suggestion

Seat your longest bullet so it is as deep as you care to make it. I know a lot of people don't like the base of the bullet extending past the neck shoulder junction if they can prevent it. This should give you the most amount of land to use as the barrel wears and make it easier to keep the bullets in contact with the lands.
 
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